Prasanna

Students can Download Chapter 1 Reproduction in Organisms Notes, Plus Two Botany Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Botany Notes Chapter 1 Reproduction in Organisms

Asexual Reproduction

The offspring shows similarity in morphological and genetical characters. They also shows resemblance to their parents i.e they are exact copies. So they are called as clone.

Binary fission in Amoeba

In Protists and Monerans, the parent cell divides into two and give rise to new individuals. In such a case cell division is the mode of reproduction. It is called binary fission (e.g., Amoeba, Paramecium).

Budinq in yeast

In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which, later separated and mature into new yeast organisms (cells).

Formation of zoospore

Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures are called zoospores (motile structures).

Other common asexual reproductive structures are conidia (eg-Penicillium), buds (eg-Hydra) and gemmules (eg-sponge).

In plants the asexual reproduction mainly by the units of vegetative propagative structures. They are

1. Runner
2. Rhizome
3. Sucker
4. Tuber
5. offset
6. bulb.

They are capable of giving rise to new offspring. These structures are called vegetative

In water bodies, aquatic plant ‘water hyacinth’ is an invasive weed, called as the ‘Terror of Bengal’. Because it can propagate and spread all over the water body in a short period of time. It depletes the amount of oxygen, which leads to death of fishes.

Another examples of vegetative propagation are the origin of new plants from the buds (called eyes) of the potato tuber, from the rhizomes of banana and ginger.

In some plants adventitious buds arise from the notches present at margins of leaves eg- Bryophyllum. So it is used for the commercial propagation of plants.

Sexual Reproduction
It involves formation of the male and female gametes of the same individual or different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism.

So the offsprings are not identical to the parents or amongst themselves. They are different in external morphology, internal structure, and physiology.

The growth period of organisms before reproduction is called the juvenile phase (vegetative phase).This phase shows variable durations in different organisms.

It is found that some plants flower throughout the year and some others that show seasonal flowering. But few plants exhibit unusual flowering phenomenon;

Bamboo It flowers only once in their life time, after 50 – 100 years, produce large number of fruits and die. Strobilanthus kunthiana (neelakuranji). It flowers once in 12 years. Recently this plant flowered hilly areas in Kerala, Karnataka, and Tamil Nadu into blue stretches during September-0ctober2006 and attracted a large number of tourists.

In animals, the juvenile phase is followed by morphological and physiological changes. But the duration of reproductive phase is varying in different organisms.

Some animals lay their eggs throughout the year but others seasonally. The females of placental mammals exhibit cyclical changes in the ovaries as well as hormones during the reproductive phase.

In non-primate mammals like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during reproduction are called oestrus cycle where as in primates (monkeys, apes, and humans) it  is called menstrual cycle.

Many mammals exhibit such cycles only during favourable seasons in their reproductive phase, they are called as seasonal breeders. The other mammals are reproductively active throughout their reproductive phase they are called continuous breeders.

The end of reproductive phase is called as senescence or old age. Later it leads to death. In both plants and animals, hormones and environmental factors regulate the reproductive processes and behavioural expressions of organisms.
Events in sexual reproduction: It involves

1. pre-fertilisation
2. fertilisation
3. post-fertilisation

1. Pre-fertilisation Events:
These include gametogenesis and gamete transfer.

a. Gametogenesis:
It is the process of formation of two types of haploid gametes – male and female.

In some algae the two gametes have similar morphology, they are called as homogametes or isogametes.

Some sexually reproducing organisms produce the gametes of two morphologically distinct types (antherozoid or sperms egg or ovum). They are called as heterogametes.

Sexuality in organisms:
Both the male and female reproductive structures are present in the same plant, they are bisexual or on different plants, they are unisexual.

Homothallic and monoecious are used to denote the bisexual condition Eg-some fungi and plants. Heterothallic and dioecious are used to denote unisexual condition.

In flowering plants, the unisexual male flower is staminate, i.e., bearing stamens, while the unisexual female flower is pistillate i.e bearing pistils.

In some flowering plants, both male and female flowers are present on the same individual (monoecious) eg- cucurbits and coconuts or on separate individuals (dioecious) eg- papaya and date palm.

Earthworms, sponge, tapeworm, and leech, etc. are bisexual animals that possess both male and female reproductive organs, they are called hermaphrodites.

Cell division during gamete formation:
Diploid parent that produces haploid gametes by reduction division meiotic division. But the haploid parent produces gametes by mitoticdivision.

Members of monera, fungi, algae, and bryophytes have haploid plant body, but pteridophytes, gymnosperms, angiosperms, and human beings have diploid parent body.

In diploid organisms, meiocytes undergo meiosis to form haploid gametes contain only one set of chromosomes.

b. Gamete Transfer:
In most organisms, male gamete is motile and the female gamete is stationary. But few fungi and algae both types of gametes are motile.

They need water as a medium through which the male gametes moves. In algae, bryophytes and pteridophytes. water is the medium through which the gamete transfer takes place.

In seed plants, pollen grains are the carriers of male gametes, and ovule have the egg. In dioecious animals the gametes are formed in different individuals and the organism have special mechanism for gamete transfer.

In bisexual, self-fertilising plants, e.g. peas, transfer of pollen grains to the stigma takes place when it come in contact with the stigma. But in cross pollinating plants pollination agency helps the transfer. Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.

2. Fertilisation:
It is the fusion of gametes. This process is called syngamy leads to the formation of a diploid zygote.

In rotifers, honeybees, some lizards and birds (turkey), the female gamete undergoes development to form new organisms without fertilisation. This phenomenon is called parthenogenesis.

In most aquatic organisms, such as algae, fishes and amphibians syngamy occurs outside the body of an organism (water) This type of gametic fusion is called external fertilisation.

For this, male partner release a large number of gametes into the surrounding medium (water). This is the case of bony fishes and frogs where a large number of offspring are produced.

One disadvantage is that the offspring are here prey, subjected to the attack of the predators. In terrestrial fungi, reptiles birds, mammals, and plants (bryophytes, pteridophytes, gymnosperms, and angiosperms), syngamy occurs inside the body of the organism, it is called internal fertilisation.

Here the male gamete is motile and has to reach the egg for fertilisation. In seed plants, the non-motile male gametes are carried to female gamete by pollen tubes.

3. Post-fertilisation Events:
It involves events afterthe formation of zygote.

a. The Zygote:
In fungi and algae, zygote develops a thick wall that is resistant to descication and damage. It undergoes a period of rest before germination.

In organisms with haplontic life cycle, zygote divides by meiosis to form haploid spores that grow into haploid individuals.

Zygote is the vital link that maintains the continuity of species between organisms of one generation and the next.

b. Embrvoaenesis:
It is the process of development of embryo from the zygote. During embryogenesis, zygote undergoes cell division and cell differentiation (modifications to form specialised tissues and organs).

In oviparous animals like reptiles and birds, the fertilised eggs are covered by hard calcareous shell, it undergoes period of incubation and young ones hatch out.

On the other hand, in viviparous animals, the zygote develops into a young one inside the body of the female organism. Afterthe period of growth, they are delivered out.

In flowering plants, the zygote is formed inside the ovule. The sepals, petals and stamens of the flower wither and fall off.

Zygote develops into the embryo. The ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp (protective).

Kinds of fruits showing seeds and pericarp

Students can Download Chapter 5 Biotechnology and its Applications Notes, Plus Two Botany Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Botany Notes Chapter 5 Biotechnology and its Applications

Biotechnological Applications In Agriculture
The important methods that useful for increasing food production are

(i)  Agro-chemical based agriculture (ii) Organic agriculture; and (iii) Genetically engineered crop-based agriculture

The Green Revolution helped to increase food production in many fold but it is not enough to meet the demand of growing human population. Here Genetically modified crops are the possible solution for this crisis.

GM (Genetically Modified) plants are useful in many ways

1. Made crops more tolerant to abiotic stresses (cold, drought, salt, heat).
2. Reduced reliance on chemical pesticides (pest-resistant crops).
3. Helped to reduce post harvest losses.
4. Increased efficiency of mineral usage by plants
5. Enhanced nutritional value of food, e.g., Vitamin ‘A’ enriched Golden rice.

Eg-Bt cotton, Bt corn, rice, tomato, potato, and soyabean, etc have a gene for resistance to insects.

Bt Cotton
Bt toxin producing cry genes are isolated from Bacillus thuringiensis and inserted into the several crop plants such as cotton. The isolation of genes depends upon the crop and the targeted pest because most Bt toxins are insect-group specific,
For example

1. crylAc and cryllAb control the cotton bollworms 2. crylAb controls corn borer

Insecticidal protein of some species of Bacillus thuringiensis that kill certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes).

Bt toxin protein exist as inactive protoxins but it is converted into an active form in the presence of the alkaline pH of insect gut. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and results in the death of insect.

Pest Resistant Plants:
Nematode Meloidegyne incognitia infects the roots of tobacco plants and causes a great reduction in yield. It is nessary to control the attack of insect pest.

The best method used to prevent the attack of nematode is RNA interference (RNAi). It involves silencing of a specific mRNA of nematode.

Here the complementary dsRNA molecule that binds to and prevents translation of the mRNA (silencing).

After the insertion of nematode-specific genes by Agrobacterium vectors into the host plant, it produce both sense and antisense RNA in the host cells. These two RNA’s being complementary to each other formed a double-stranded (dsRNA) that initiated RNAi and silenced the specific mRNA of the nematode.

Biotechnological Applications In Medicine
The recombinant DNA technological processes that helpful in the mass production of safe and more effective therapeutic drugs.

In world, about 30 recombinant therapeutics are marketed for human-use. In India, 12 of these are presently being marketed.

1. Genetically Engineered Insulin:
Insulin for diabetes was extracted from pancreas of slaughtered cattle and pigs, it caused allergic disease in some patients. In humans, insulin is synthesised as a prohormone which contains an extra stretch called the C peptide. It is removed and converted into a fully mature and functional insulin.

It consists of two short polypeptide chains: chain A and chain B, that are linked together by disulphide bridges.

An American company Eli Lilly in 1983 prepared two DNA sequences corresponding to A and B, chains of human insulin, and inserted in plasmids of E. coli to produce insulin chains. Chains A and B were produced separately, extracted, and combined by creating disulfide bonds to form human insulin.

2. Gene Therapy:
It is the replacement of defective gene by functional gene. This is done by transferring the functional gene into the individual cells, tissues or embryo to treat a disease.

The first reported case of gene therapy was adenosine deaminase (ADA) deficiency that seriously affected the functioning of the immune system. It is due to the deficiency of gene for adenosine deaminase.

Before genetic engineering, ADA deficiency cured by bone marrow transplantation or enzyme replacement therapy.

In the first step of gene therapy, lymphocytes from the blood of the patient are cultured and functional ADA cDNA is introduced in it. Then, these cells are return back to the patient. So that the patient requires frequently such genetically engineered lymphocytes.

The permanent cure for such disease is to introduce functional ADA cDNA into cells at early embryonic stages.

3. Molecular Diagnosis:
Early diagnosis of disease is possible by

1. Recombinant DNA technology 2. Polymerase Chain Reaction (PCR) and 3. Enzyme Linked Immuno-sorbent Assay (ELISA)

Low concentration of a bacteria or virus can be detected by amplification of their nucleic acid by PCR. It is used to detect HIV in suspected AIDS patients and detect mutations in suspected cancer patients It is also used to identify genetic disorders.

The presence of mutated gene can be detected by a probe. A single stranded DNA or RNA, tagged with a radioactive molecule. It is then hybridise to its complementary DNA in a clone of cells. By using autoradiography it is observed that the probe not have any complimentarity with the mutated gene.

Transgenic Animals
Out of many transgenic animals such as rats, rabbits, pigs, sheep, cows fish, etc. 95 percent of transgenic animals are mice.
Importance of such animals are
(i) Normal physiology and development:
Transgenic animals can be used to study of how genes are regulated, and how they affect the normal functions of the body and its development, e.g., study of insulin-like growth factor.

(ii) Study of disease:
Transgenic animals can be used to know, how genes contribute to the development of disease. Today transgenic models exist for many human diseases such as cancer, cystic fibrosis, rheumatoid arthritis and Alzheimer’s disease.

(iii) Biological products:
Transgenic animals that produce useful biological products. For example the introduction of genes which codes for a particular product such as human protein (-antitrypsin) used to treat emphysema. Another examples of disease treated are phenylketonuria (PKU) and cystic fibrosis.

In 1997, the first transgenic cow, Rosie, produced human protein-enriched milk (2.4 grams per litre). The milk contained the human alpha-lactalbumin. It is nutritionally a more balanced product for human babies than natural cow-milk.

(iv) Chemical and vaccine safety testing:
Transgenic animals carry genes that sensitive to toxic substances than non-transgenic animals. So they are exposed to the toxic substances and the effects studied.

Ethical Issues
Some ethical standards are maintained to evaluate the morality of all human activities that are either useful or harmful because the genetically modified organisms have unpredictable results.

Government of India has set up organisations such as GEAC (Genetic Engineering Approval Committee), they take decisions regarding the validity of GM research and the safety of introducing GM-organisms for public services.

Today the patents are given for products and technologies that make use of the genetic materials, plants, and other biological resources, that have long been identified, developed, and used by farmers and indigenous people of a specific region/country. This is an important problem.

For example, it is estimated that 200,000 varieties of rice grown in India. Of which Basmati rice is distinct for its aroma and flavour. It is significant because this variety was referred in ancient texts, folklore, and poetry.

In 1997, an American company got patent rights on Basmati rice. It was helped the company to sell a ‘new’ variety of Basmati in the US and abroad. It is derived from Indian farmer’s varieties. But the patenting procedure restricts the selling and exporting of Basmati rice by other countries.

Similar attempts have also been made to patent uses, products, and processes based on Indian traditional herbal medicines, e.g., turmeric neem.

Therefore it is necessary to resist these patent applications of other countries/individuals because they permanently take overfull control of our resources.

Biopiracy

It is the unauthorised use of bio-resources by multinational companies and other organisations without compensatory payment.

Industrialised nations are financially rich but poor in biodiversity and traditional knowledge But the developing nations is rich in biodiversity and traditional knowledge related to bio-resources.

Here the sharing between developed and developing countries for traditional knowledge related to bio-resources has not been take place on the basis of compensatory payment. Therefore, some nations are developing laws to prevent such unauthorised exploitation of their bio-resources and traditional knowledge.

Recently Indian Parliament cleared the second amendment of the Indian Patents Bill, that takes such issues related to patents.

Students can Download Chapter 11 Alcohols, Phenols and Ethers Notes, Plus Two Chemistry Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Chemistry Notes Chapter 11 Alcohols, Phenols and Ethers

Alcohols and phenols are formed when a hydrogen atom in hydrocarbon is replaced by -OH group. In alcohols one or more -OH groups are directly attached to carbon atom(s) of an aliphatic system. While phenols contain -OH group(s) directly attached to carbon atom(s) of an aromatic system. Ethers are alkoxy oraryloxy hydrocarbons.

Classification
1. Mono, Di, Tri or Polyhydric Compounds:
Alcohols and phenols are classified as mono-di-tri-polyhydric depending upon number of -OH group.

(i) Compounds Containing sp³ – C – OH Bond:
The – OH group is attached to sp³ C. They are further classified as 1°, 2°, and 3°.

Allylic Alcohols:
The -OH group is attached to a sp³ C next to the C = C.

Benzylic Alcohols:
The -OH group is attached to a sp³ C next to an aromatic ring.

Allylic and benzylic alcohols may be 1°,2° or 3°.

(ii) Compounds Containing sp² C- OH Bond:
The – OH group bonded to a C = C i.e., to a vinylic or to an aryl C. Vinylic alcohol: CH₂ = CH – OH

Ethers:
They are classified as simple ethers or symmetrical ethers – if the alkyl or aryl group attached to O are same and mixed ether or unsymmetrical ether- if the two groups attached to O are different.
Simple ethers
CH₃ – O – CH₃
C₂H₅ – O – C₂H₅
Mixed ethers
CH₃ – O – C₂H5₃
C₂H₅ – O – C₃H₇

Nomenclature
(a) Alcohols: Common name – alkyl alcohols
IUPAC – alkanols (‘e’of the alkane is replaced by ‘ol’)

(b) Phenols:
These are hydroxy derivative of benzene. The name phenol is also accepted by IUPAC.

(c) Ethers:
Common name – Alkyl Ether
IUPAC name – Aikoxyalkane
The smaller R – group is chosen as alkoxy and larger R – group is choosen as parent hydrocarbon.

Structure of Functional Group
Alcohols:
C of C – OH bond is sp³ hybridised. Bond is formed by sp³ – sp³ overlap. C – OH bond angle slightly less than the tetrahedral bond angle(109°28’) due to the repulsion between the unshared electron pairs of oxygen.

Phenols:
The -OH group is attached to sp² C of an aromatic ring. The C – O bond length is slightly less than that in methanol. This is due to

1. Partial double bond character on account of the conjugation of unshared electron pair of O with the aromatic ring and
2. sp² hybridised state of C to which O is attached.

Ethers:
The 4 electron pairs (2 bond pairs and 2 lone pairs) on O are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greaterthan the tetrahedral angle due to the repulsive interaction between the two bulky -R groups.

Alcohols and Phenols
1. Preparation of Alcohols
(1) From alkenes:
(i) By acid catalysed hydration:
Alkenes react with water in presence of acid as catalyst. The addition is according to Markovnikov’s rule.

(ii) By hydroboration – oxidation:
Diborane – B₂H₆ or (BH₃)₂ reacts with alkene to given trialkyl borane which is oxidised to alcohol by H₂O₂ in presence of aq. NaOH.

(2) From Carbonyl Compounds:
(i) By Reduction of Aldehydes and Ketones:
These are reduced using LiAlH₄, NaBH₄, H₂Pd etc.
Aldehyde gives 1° alcohols, while ketones gives 2° alcohols.

(ii) By the Reduction of Carboxylic Acids or Esters:

(3) From Griguard Reagents:
By the reaction of Grignard reagents with aldehydes and ketones. The adduct formed by the nucleophilic addition of RMgX to carbonyl group on hydrolysis yeilds alcohol.

• Formaldehyde (HCHO) gives 1° alcohols
• Other aldehydes (R – CHO) give 2° alcohols
• Ketones (R – CO – R) give 3° alcohols

Example:

2. Preparation of Phenols:
(a) From Hatoarenes:

(b) From Benzene Sulphonic Acid:

(c) From Diazonium Salts:

(d) From Cumene (Isopropylbenzene):

3. Physical Properties:
The boiling points of alcohols and phenols increase with increase in the number of carbon atoms. In alcohols the boiling points decrease with increase of branching in carbon chain. The high boiling point of alcohols is due to intermolecular hydrogen bonding.

Solubility:
Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonding. The solubility decreases with increase in size alkyl/aryl group.

4. Chemical Reactions of Alcohols:
(a) Reactions Involving Cleavage of O-H Bond
(1) Acidity of Alcohol and Phenols:
(i) Reaction with Metals:
Alcohols and phenols react with active metals such as Na, K and Al to yield corresponding alkoxides/phenoxides and H₂.

Phenols react with aq. NaOH to form sodium phenoxides.

(ii) Acidity of Alcohols:
It is due to the polar nature of O-H bond. Electron releasing groups increase electron density on O tending to decrease the polarity of O-H bond. The acid strength of alcohols decreases in the order: 1°> 2° > 3° alcohols.

(iii) Acidity of Phenols:
The reaction of phenol with aqueous NaOH indicates that phenols are stronger acids than alcohols and water. Acidity of phenols can be explained by resonance.

The delocalisation of negative charge makes phenoxide ion more stable and favours the ionisation of phenol.

The acidity of phenols increases if an electron-withdrawing group is present at 0- and p- position. Electron releasing groups decrease the acidity.

(2) Esterification:
Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.

Acefy/afron:
Introduction of acetyl (CH₃CO-) group in alcohols or phenols. Acetylation of salicylic acid produces aspirin.

(b) Reaction Involving Cleavage of C-0 Bond in Alcohols 1) Reaction with Hydrogen Halides:
R – OH + HX → R – X + H₂O
(1) Lucas Test:
Alcohols are distinguished by Lucas reagent (cone. HCI and ZnCl₂). On treating with Lucas reagent, 3° alcohol gives immediate turbidity, 2° alcohol gives turbidity after few minutes, 10 alcohol do not produce turbidity at room temperature.

(2) Reaction with Phosphorus Trihalide (PCl₃):
3 R – OH + PCl₃ → 3 R – Cl + H₃PO₃.

(3) Dehydration:
Alcohols undergo dehydration to form alkenes on treating cone. H₂SO₄ or H₃PO₄.

e.g. ethanol undergoes dehydration by heating it with cone. H₂SO₄ at 443 K.

The relative ease of dehydration of alcohols in the follows the order 3° > 2° > 1°

(4) Oxidation:
It i nvolves the formation of a

1° alcohols are oxidised to aldehydes.

Strong oxidising agents such as acidified KMnO₄ or K₂Cr₂O₇ are used forgetting carboxylic acids from alcohols directly. A better reagent for oxidation of 1° alcohol to aldehydes is pyridinium chlorochromate (PCC).

2° alcohols are oxidised to ketones by CrO₃.

3° alcohols do not undergo oxidation.

Dehydrogenation:
When the vapours of a alcohols are passed over heated Cu at 573 K,
1° alcohols give addehyde.

2° alcohols give ketones.

3° alcohols undergo dehydration to give alkene.

(c) Reactions of Phenols:
(1) Electrophilic Aromatic Substitution:
The -OH group attached to the benzene ring activates it towards electrophilic substitution. It is 0- and p- directing.
(i) Nitration:

o-Nitrophenol is steam volatile due to intramolecular H – bonding and p-Nitrophenol is less volatile due to intermolecular H -bonding. Hence the mixture can be seperated by steam distillation.

(ii) Halogenation:

(2) Kolbe’s Reaction:
Phenol, when treated with NaOH, forms sodium phenoxide which undergoes electrophilic substitution with CO₂ to give 2- Hydroxybenzoic acid (Salicylic acid).

(3) Reimer – Tiemann Reaction:
On treating phenol with CHCI3 in presence of aq. NaOH, 2 – Hydroxy benzaldehyde (Salicylaldehyde) is formed.

(4) Reaction with Zn Dust:

(5) Oxidation:

Some Commercially Important Alcohols
(1) Methanol (CH₃ – OH):
It is also known as wood spirit. Industrial preparation – Catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO-Cr₂O₃ catalyst.

Methanol is a colourless liquid, poisonous in nature, cause blindness and in large quantities causes even death. It is used as solvent in paints and varnishes.

(2) Ethanol (C₃ – CH₂ – OH):
Obtained commercially by fermentation of sugars. The enzyme invertase present in the yeast converts sugar into glucose and fructose, which undergo fermentation in presence of zymase, another enzyme found in yeast.

It is a colourless liquid, used in paint industry as a solvent.
Rectified spirit – 95.6% ethanol.
Absolute alcohol – Pure anhydrous alcohol (100% alcohol)
Power alcohol – Alcohol mixed with gasoline (1:4 ratio).
Denatured spirit- The commercial alcohol is made unfit for drinking by mixing in it some CuSO₄, pyridine or methanol. It is known as denaturation of alcohol.

Ethers
1. Preparation of Ethers:
(1) By Dehydration of Alcohols:
Alcohols undergo dehydration in presence of protic acids. (H₂SO₄, H₃PO₄)

(2) Williamson Synthesis:
Alkyl halides react with sodium alkoxide to form ether.
R – X + R’ONa → R – O – R’ + NaX
Better results are obtained if alkyl halide is primary.

In case of secondary and tertiary akyl halides, elimination competes substitution.

Phenols are also converted to ethers by this method.

2. Physical Properties:
Ethers have much lower boiling points than the alcohols. It is due to the presence of H-bonding in alcohols. Lower members of ethers are soluble/miscible in water as they form hydrogen bonds with water molecule.

3. Chemical Reactions:
(1) Cleavage ofC-0 Bond in Ethers:
It takes place under drastic conditions with excess of HX.
R – O – R + HX → R – X + R – OH
R – OH + HX → R – X + H₂O
Alkyl aryl ethers react with HX to give phenol and alkyl halide.

Order of reactivityof hydrogen halides is Hl > HBr > HCI. In the reaction of ether with HI, if the ether contains primary or secondary alkyl groups, it is the lower alkyl group that forms alkyl iodide.

e.g. CH₃ – O – CH₂CH₃l + H – l → CH₃l + CH₃CH₂OH. When one of the alkyl group is a tertiary group, the halide formed is a tertiary halide.

(2) Electrophilic Substitution:
It occurs at o- and p- position as the -OR group is o- and p- directing.
(i) Hologenation:

(ii) Nitration:
Anisole reacts with cone. HN03 as follows:

(iii) Friedel-Crafts reaction:
(a) Alkylation:

(b) Acetylation:

Students can Download Chapter 2 Inverse Trigonometric Functions Notes, Plus Two Maths Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Maths Notes Chapter 2 Inverse Trigonometric Functions

Introduction
Trigonometric functions are real functions which are not objective and thus its inverse does not exist. In this chapter we study about the restrictions on domains and ranges of trigonometric functions which ensure the existence of their inverse and observe its graphical peculiarities.

A. Concepts
I. Functions
sin^-1 x : [-1, 1 ] → [-\(\frac{\pi}{2}\), \(\frac{\pi}{2}\)]
cos^-1 x: [-1, 1] → [0, π]
tan^-1 x : R → \(\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)\)
cosec^-1 x : R – (-1, 1) → [-\(\frac{\pi}{2}\), \(\frac{\pi}{2}\)] – {0}
sec^-1 x : R -(-1, 1) → [0, π] – {\(\frac{\pi}{2}\)}
cot^-1 x : R → (0, π)

II. Properties
1. sin (sin^-1 x) = x, x ∈ [-1, 1]
sin^-1(sinx) = x, x ∈ [-\(\frac{\pi}{2}\), \(\frac{\pi}{2}\)]
cos(cos^-1 x) = x, x ∈ [-1, 1]
cos^-1(cosx) = x, x ∈ [o, π]
tan(tan^-1 x) = x, x ∈ R
tan^-1(tan x) = x, x ∈ \(\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)\)

2. sin^-1(-x) = -sin^-1(x), x ∈ [-1, 1]
tan^-1(-x) = -tan^-1(x), x ∈ R
cosec^-1(-x) = -cosec^-1(x), x ∈ R -(-1, 1)
cos^-1(-x) = π – cos^-1(x), x ∈ [-1, 1]
cot^-1(-x) = π – cot^-1(x), x ∈ R
sec^-1(-x) = π – sec^-1(x), x ∈ R -(-1, 1)
sin^-1(x) + cos^-1(x) = \(\frac{\pi}{2}\), x ∈ [-1, 1].

3. cosec^-1(x) + sec^-1(x) = \(\frac{\pi}{2}\), |x| ≥ 1
tan^-1(x) + cot^-1(x) = \(\frac{\pi}{2}\), x ∈ R

4. sin^-1 x

5. cos^-1 x

6. tan^-1(x) + tan^-1(y) =

7. tan^-1(x) – tan^-1(y) =

8. 2 tan^-1 x

9. sin^-1 x ± sin^-1 y

Students can Download Chapter 13 Nuclei Notes, Plus Two Physics Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Physics Notes Chapter 13 Nuclei

Introduction
In this chapter, We shall discuss various properties of nuclei such as their size, mass and stability, and also associated nuclear phenomena such as radioactivity, fission and fusion.

Atomic Masses And Composition Of Nucleus
1. Atomic Mass Unit (amu or u):
The most commonly used unit to express atomic mass of nucleus is atomic mass unit (u). It is defined as 1/12^th of mass of carbon atom (C¹²).

2. Proton:
The nucleus of lightest atom (isotope) of hydrogen is called proton. The mass of proton is
m_p = 1.00727u = 1.67262 × 10^-27 kg
The charge of proton is +1.6 × 10^-19 and it is stable.

3. Discovery of Neutron:
Neutron was discovered by James Chadwick. He bombarded Beryllium nuclei with α particles and observed the emission of neutral radiation. He assumed the neutral radiation consists of neutral particles called neutron.

4. Neutron:
Neutron is changeless particle of mass 1.6749 × 10^-27kg. Neutron is stable inside nucleus but it is unstable in its free state.

5. Representation of Nuclide:
Nuclear species or nuclides are represented by notation ^A_ZX, where X is the chemical symbol of species.
Z → Atomic Number = Number of protons (electrons)
N → Neutron Number = Number of neutrons
A → Mass Number = Z + N (Total number of protons and neutrons)

6. Isotopes, Isobars and Isotones:

Size Of The Nucleus
The radius of nucleus is related to mass number (A) by the equation
R = R₀A^1/3
where ₀ = 1.2 × 10^-15 m
The volume of nucleus (the shape of nucleus is assumed to be spherical) is proportional to A.
ie. Volume = \(\frac{4}{3}\)πR³ = \(\frac{4}{3}\)R₀³.A
∴ Volume α A
The density of nucleus is constant. It is independent of A and its value is 2.3 × 10¹⁷kgm^-3

Mass Energy And Nuclear Binding Energy
1. Mass Energy:
According to Einstein mass is considered as a source of energy. The mass ‘m’ can be converted into energy according to relation
E = mc²
This is mass energy equivalence relation. C is the velocity of light (3 × 10⁸m/s).

2. Nuclear binding energy:
(A) Mass Defect:
The mass defect (Am) is the difference in the mass of nucleus and total mass of constituent nucleons.
∆m = (ZM_P + (A – Z)m_n] – M
m_P and m_n are mass of proton and neutron respectively. M is the mass of nucleus.
Eg: In ¹⁶₈O, there are 8 protons and 8 neutrons. The atomic mass of ¹¹₈O is 15.99493u. The expected mass of ¹⁶₈O is sum of masses of its nucleons.
Total mass of nucleons
= 8 × m_P + 8 × m_n
= 8 × 1.00727u + 8 × 1.00866u
= 16.12744u
The difference in mass,
∆m = 16.12744u – 15.99493u = 0.13691u

(B) Binding Energy and Binding Energy per nucleon (E_b and E_bn):
Binding Energy: Mass defect (∆m) gets converted into energy as
E_b = ∆mc²
This energy is called binding energy. Which binds nucleons inside the nucleus.

Binding Energy per nucleon:
Binding energy per nucleon E_bn is the ratio of binding energy of nucleus to number of nucleons
E_bn = \(\frac{E_{b}}{A}\)
E_bn is the measure of stability of nucleus.

(C) Plot of E_bn versus mass number, A Main features of the graph:

• E_bn is almost constant for nuclei whose mass number ranges as 30 < A < 170. The maximum value of E_bn is 8.75Mev for ⁵⁶Fe and it is 7.6MeV for ²³⁸U.
• E_bn is low for lighter nuclei and also for heavier nuclei.
• There appear narrow spikes in the curve.

The conclusions from the features of graph:

• The force is attractive and sufficiently strong.
• The nuclear force is short range. Each nucleon has its influence on its immediate neighbors only so nuclear force is saturated.
• Heavier nuclei like U²³⁸ have low E_bn. So it split up into nuclei of high Ebn releasing energy ie. it undergoes fission.
• Lighter nuclei like ²H, ³H, etc. have low E_bn. So it combine to form a heavier nuclei of high E_bn releasing energy ie. it undergoes nuclear fusion.
• The nuclei at the peaks of narrow spikes have high E_bn which shows extra stability.

Nuclear Force
The features of nuclear force are:

1. The nuclear force is the strongest force in nature.
2.  The nuclear force is saturated. It is short range force.
3. The nuclear force is charge independent ie. nuclear force between proton-proton, neutron-neutron, and proton-neutron are the same.

Variation of potential energy with distance:
The potential energy of a pair of nucleons as a function of their separation is shown in the figure


At a particular distance r₀, potential energy is minimum. The force is attractive when r > r₀ and it is repulsive when r < r₀. The value of r0 is about 0.8fm.

Radioactivity
A.H. Becquerel discovered radioactivity.
In radioactive decay, unstable nucleus undergoes decay into stable one. There are three types of decay

1. α decay
2. β decay
3. γ decay

1. Law of Radioactive Decay:
According to Law of Radioactive decay, the number of nuclei undergoing decay per unit time (or rate of decay) is proportional to number of nuclei in the sample at that time.

λ is decay constant or disintegration constant. The negative sign indicates that number of nuclei is decreasing with time. The solution to the above differential equation is
N = N₀e^-λt
N₀ is the initial number of atoms. This equation shows that number of nuclei is decreasing exponentially with time as shown below.

Derivation of equation N(t) = N(0)e^-λt
According to Law of Radioactive decay,
\(\frac{d N}{d t}\) = -λn
\(\frac{d N}{d t}\) = -λdt
Integrating
InN = -λt + C_____(1)
C is the constant of integration. To get value of C, let us assume that initially (t = 0) the number of nuclei be N₀
∴ C = In N₀
Substituting for C in equation (1) we get,
InN – In N₀ = -λt
In\(\frac{N}{N_{0}}\) = -λt
\(\frac{N}{N_{0}}\) = e^-λt
N = N₀e-λt

(A) The decay rate (R):
The decay rate is number of nuclei disintegrating per unit time and is denoted by R.
R = \(\frac{-d N}{d t}\)
Differentiating the equation N = N₀e^-λt, we get

In terms of decay rate we get R = R₀e^-λt
where R₀ = λN₀, decay rate at t = 0

(B) Half life (T_1/2):
It is the time taken by radio nuclide to reduce half of its initial value.
half life period T_1/2 = \(\frac{0.693}{\lambda}\)
Relation between (T_1/2) and λ
If T_1/2 is the half-life period, then N = \(\frac{\mathrm{N}_{0}}{2}\)
Substituting these values in N = N₀e^-λt, we get,
\(\frac{\mathrm{N}_{0}}{2}\) = N₀e^-λT_1/2
2 = e^-λT_1/2
Taking log on both sides we get,
log_e2 = λT_1/2 (since log e^x = x)

(C) Mean life(t) or average life:
It is defined as time taken by radio nuclei to reduce 1/e^th of its initial value.
Mean life τ = \(\frac{1}{\lambda}\)
proof
We know In (\(\frac{N}{N_{0}}\)) = -λt

∴ t = τ, N = \(=\frac{N_{0}}{e}\)
In(1/e) = -λτ
In(e) = λτ
In e = 1
1 = λτ
∴ τ = 1/λ

(D) Relation between τ and T_1/2
T_1/2 = 693τ

(E) Units of Radioactivity:
The SI unit for radio activity is Becquerel. One becquerel is one disinte¬gration per second. The traditional unit of activity is curie.
1 curie = 3.7 × 10¹⁰ Bq

2. Alpha Decay (α decay):
In α decay, mass number is reduced by 4 units and atomic number is reduced by 2 units.

Q-value
Q value is the energy released in nuclear reaction. The Q value or disintegration energy of a decay can be defined as the difference between the initial mass energy and final mass energy of decay products The Q value of a decay is expressed as
Q = (m_x – m_y – m_He)c²

3. Beta decay (β – decay): There are two types of β decay

• β⁺ decay
• β^– decay

a. β⁺ decay:
In β⁺ decay atomic number is reduced by 1 unit. But mass number remains unchanged.

In β⁺ decay, positron and neutrino are emitted. In β⁺ decay, conversion of proton into neutron, positron and neutrino takes place.

b. β^– decay:
In β^– decay, atomic number is increased by 1 unit. But mass number does not change.

In β^– decay a neutron converts into proton emitting electron and antineutrino.

4. Gamma Decay:
The excited nucleus comes back to ground state by emitting gamma rays.
Eg:

5. Properties of α, β and γ:
Properties of α – particle:

• α -particles have a charge of +2e and a mass four times that of hydrogen atom.
• They are deflected by electric and magnetic fields.
• They affect photographic plates.
• They produce fluorescence and phosphorescence.
• They have a high ionizing power.
• They can penetrate very thin metal foils.
• The velocity is of the order of 10⁷ m/s.

Properties of β – particles:

• β – particle is an electron.
• They are deflected by electric and magnetic fields.
• They can affect photographic plates.
• They can produce fluorescence and phosphorescence
• They have low ionization power.

Properties of γ – ray:

• γ – rays are electromagnetic waves.
• They have the speed of light.
• They have high penetrating power.
• They can affect photographic plates.
• They can produce fluorescence and phosphorescence.
• They have ionizing power.
• They are not deflected by electric and magnetic fields.

Nuclear Energy:
In the nuclear reactions, huge quantity of energy is released

1. Fission:
In nuclear fission, a heavier nuclei split into lighter ones releasing huge energy. When Uranium atom is bombarded with neutron, it breaks into intermediate mass fragments as shown.

Note:

• The energy released perfission of Uranium nucleus is 200MeV.
• The neutrons released per fission of Uranium nucleus is 2.5
• Controlled chain reaction (nuclear fission) is basic principle of nuclear reactor.
• Uncontrolled chain reaction results in explosion. This is the principle behind atom bomb.

A. Chain reaction:
The nuclear fission (of U²³⁸) produces extra neutrons. These extra neutrons may bombard with the neighboring Uranium atoms and make it to undergo nuclear fission.

This fission again produces more neutrons. This process continues like a chain. This was first suggested by Enrico Fermi.

2. Nuclear Reactor:
The controlled chain reaction produce a steady energy output. This is the basic of nuclear reactor.
The main components of nuclear reactor:

(i) Fissionable material or fuel:
The fissionable material is (²³⁵₉₂U). Which is placed inside the core where the fission takes place.

(ii) Moderator:
It is used to slow down fast moving neutron. Commonly used moderators are water, heavy water (D₂O), and graphite.

(iii) Reflector:
The core is surrounded by reflector to prevent the leakage.

(iv) Control rods:
Its purpose is to absorb neutron and hence to control reaction rate. It is made up of neutron-absorbing material like Cadmium.

(v) Coolant:
The energy released in the form of heat is continuously removed by coolant. It transfers heat to the working fluid.

The whole assembly is properly shielded to prevent radiation from coming out. The working fluid gets converted into steam by heat and it drive turbines and generate electricity.

A. Multiplication Factor (K)
Multiplication factor is a measure of growth rate of neutrons. For steady power operation, value of K should be 1. (called critical stage). If K > 1, reaction rate increases exponentially.

3. Nuclear Fusion – Energy Generation in stars:
In nuclear fusion lighter nuclei combine to form heavier nuclei releasing energy. Nuclear fusion is thermo nuclear reaction. It occurs at high temperature. At high temperature, particles get enough kinetic energy to overcome Coulomb repulsion.

Thermonuclear fusion is the source of energy in sun. The fusion inside sun involves burning of hydrogen into Helium.

4. Controlled Thermonuclearfusion:
In future, we expect to build up fusion reactors to generate power. For this to happen, the nuclear fuel must be kept at a temperature 10⁸K.

At this temperature fuel exists in plasma state. The problem is that no container can stand such a high temperature. Several countries around world including India are developing techniques to solve this problem.

Students can Download Chapter 1 Relations and Functions Notes, Plus Two Maths Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Maths Notes Chapter 1 Relations and Functions

Introduction
A relation from a non-empty set A to a non-empty set B is a subset of A × B. In this chapter we study different types of relations and functions, composition of functions, and binary operations.

Basic Concepts
I. Types of Relations:
Here we study different relations in a set A

Empty Relation:
R : A → A given by R = Φ ⊂ A × A.

Universal Relation:
R : A → A given by R = A × A.

Reflexive Relation:
R : A → A with (a, a) ∈ R, ∀a ∈ A.

Symmetric Relation:
R : A → A with
(a, b) ∈ R ⇒ (b, a) ∈ R, ∀a, b ∈ A.

Transitive Relation:
R : A → A with (a, b) ∈ R and (b, c) ∈ R ⇒ (a, c) ∈ R,
∀a, b, c ∈ A.

Equivalence Relation:
R : A → A which is Reflexive, Symmetric, and T ransitive.

Equivalence Class:
Let R be an Equivalence Relation in a set A. If a ∈ A, then the subset {x ∈ A, (x, a) ∈ R} of A is called the Equivalence Class corresponding to ‘a’ and it is denoted [a].

II. Types of Functions
One-One or Injective function.
A function f : A → B is said to be One-One or Injective, if the image of distinct elements of A under fare distinct.
i.e; f(x₁) = f(x₂) ⇒ x₁ = x₂
Otherwise f is a Many-One function.

1. Graphical approach:
If lines parallel to x-axis meet the curve at two or more points, then the function is not one-one.

Onto or Surjective function:
A function f : A → B is said to be Onto or Surjective, if every element of B is some image of some elements of A under f.
ie; If for every element y ∈ Y then there exists an element x in A such that f(x) = y.

Bijective function:
A function f : A → B is said to be Bijectiveit it is both One-One and Onto.

Composition of Functions.
Let f : A → B and g : B → C be two functions. Then the composition of f and g denoted by is gof defined
gof : A → C and gof (x) = g(f(x)).

1. If f : A → B and g : B → C are One-One, then gof : A → C is One-One.
2. If f : A → B and g : B → C are Onto, then gof : A → C is Onto.
3. If f : A → B and g : B → C are Bijective, ⇔ gof : A → C is Bijective.

Inverse Function:
If f : A → B is defined to be invertible, if there exists a function g : B → A such that gof = I_A and
fog = I_B. The function g is called the inverse of ‘f’ and is denoted by f^-1.

1. If function f : A → B is invertible only if f is bijective.
2. (gof)^-1 = f^-1og^-1.

III. Binary Operations
A binary operation ‘*’ on a set A is a function * : A × A → A, defined by a * b, a, b ∈ A.

1. * : A × A → A is commutative if a * b = b * a, ∀a, b ∈ A.
2. * : A × A → A is associative if a * (b * c) = (a * b) * c, ∀a, b, c ∈ A.
3. e ∈ A is the identity element for the binary operation * : A × A → A if a * e = a = e * a, ∀a ∈ A.
4. An elements a ∈ A is invertible for the binary operation * : A × A → A, if there exists an element b ∈ A such that a * b = e = b * a, where ‘e’ is the identity element for the operation ‘*’. Then ‘b’ is denoted by a^-1.

Students can Download Chapter 15 Communication Systems Notes, Plus Two Physics Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Physics Notes Chapter 15 Communication Systems

Introduction
The aim of this chapter is to introduce the concepts of communication, namely the mode of communication, the need for modulation, production, and detection of amplitude modulation.

Elements Of A Communication System
Every communication system has three essential elements.

1. Transmitter
2. Medium/channel
3. Receiver

The general form of a communication system is given below.

1. Transmitter:
A transmitter transmits the information after modifying it to a form, suitable for transmission.
Transducers:
Transducers is a device, which convert a physical quantity (called information) into electrical signal are known as transducers.
Examples:
microphone (convert sound into electrie signals), photodetector (convert light into electric signals) are the examples of transducers.

2. Medium/Channel:
Channel is the physical me-dium, which connects transmitter and receiver.
In the case of telephony, communication channel is the transmission lines. In radio communication (orwireless communication) the free space serves as the communication channel.

3. The receiver:
The receiver receives the transmitted signal. The received signal is converted to suitable form and deliver it to the user.

Modes of communication:
There are two basic modes of communication:

1. point-to-point
2. Broadcast

1. Point-to-Point:
In point-to-point communication mode, communication takes place over a link between a single transmitter and a receiver.
Example: Telephone communication

2. Broadcast:
In broadcast mode, there are a large number of receivers corresponding to a single transmitter.
Example: Radio and Television.

Basic Terminology Used In Electronic Communication Systems
It would be easy to understand the principles underlying any communication, if we get knowledge about the following basic terminology.
(i) Transducer:
Any device that converts one form of energy into another can be termed as a transducer.

(ii) Signal:
Information converted in electrical form and suitable for transmission is called a signal. Signals can be either analog or digital. Analog signals are continuous variations of voltage or current.

They are essentially single-valued functions of time. Sine wave is a fundamental analog signal. Sound and picture signals in TV are analog in nature.

Digital signals are those which can take only discrete step wise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a low level and ‘1’ corresponds to a high level of voltage/current.

(iii) Noise:
Noise refers to the unwanted signals that disturb communication system.

(iv) Transmitter:
A transmitter processes message signal to make it suitable for transmission.

(v) Receiver:
A receiver extracts the desired message signals from the received signals.

(vi) Attenuation:
The loss of strength of a signal while propagating through a medium is known as attenuation.

(vii) Amplification:
It is the process of increasing the amplitude of a signal using an electronic circuit called the amplifier.

(viii) Range:
It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

(ix) Bandwidth:
Bandwidth refers to the frequency range over which an equipment operates.

(x) Modulation:
The original low frequency message signal cannot be transmitted to long distances because of reasons given in Section 15.7. Therefore, the low frequency message signal is superimposed on a high frequency wave, (which acts as a carrier of the information). This process is known as modulation.

(xi) Demodulation:
The process of retrieval of information from the carrier wave is termed demodulation. This is the reverse process of modulation.

(xii) Repeater:

A repeater is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver.

Repeaters are used to extend the range of a communication system as shown in figure. A communication satellite is essentially a repeater station in space.

Bandwidth Of Signals
The bandwidth of a message signal refers to a band of frequencies, which are necessary for transmission of the information contained in the signal.

The band Of 2800 Hz (300 Hz – 3100 Hz) is enough to transmit speech signals. To transmit music, 20 KHz. band width is required (because of the high frequencies produced by the musical instruments).

Bandwidth of square wave:
A rectangular wave can be decomposed into a superposition of sinusoidal waves of frequencies ν₀, 2ν₀, 3ν₀, 4ν₀………nν₀, where n is an integer extending to infinity.

To produce a rectangular wave, we need to super impose all the harmonics ν₀, 2ν₀, 3ν₀, ………nν₀, which implies that bandwidth required for the transmission of rectangular wave is infinite.

For practical purpose, the higher harmonicas are removed. Thus bandwidth is limited. The removal of higher harmonics dos not effect the shape of rectangular wave. Because the contribution of higher harmonics to rectangular wave form is less.

Bandwidth Of Transmission Medium
Different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space and fiber optic cable.

Cable offers a bandwidth of 750MHz Optical fibre offers a bandwidth of 1 THz to 1000 THz (Microwaves to ultraviolet).

Propagation Of Electro Magnetic Waves
When the em wave travels through space, the strength of wave decreases.

1. Ground wave:
It is a mode of propagation in which the ground waves progress along the surface of the earth. As the groundwave passes over the surface of the earth, it is weakened as a result of the energy absorption by the surface.

Due to this loss the ground waves are not suited for very large range communication. The ground wave propagation is effective only in very low frequencies (VLF) 500 KHz to 1500 KHz.

2. Sky waves:
It is that mode of wave propagation in which the radiowaves emitted from the transmitting antenna reach the receiving antenna after reflection in the ionosphere.

The UV and other high energy radiations coming from sun are absorbed by air molecules. Due to this absorption, the air molecules get ionized and form an ionized layer of electrons and ions around the earth. The ionosphere extends from a height of nearly 80 Km to 300 km above the earth’s surface.

Explanation for reflection of em wave:
The refractive index of ionosphere decreases as we go into the ionosphere. Therefore an em wave coming from ground undergoes fortotal internal reflection.

Since this phenomenon is a frequency dependent one, there is a critical frequency (ranges from 5 to 10 MHz) above which the wave incident on the ionosphere will not reflect back. Therefore, sky wave propagation is not possible above 10 MHz. This limitation is overcome with satellite communication.

3. Space wave:
A space wave travels in a straight line from transmitting antenna to the receiving antenna. Space wave communication is also called Line of sight (LOS) communication.

Because of line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth as illustrated in the above figure.

If the signal is to be received beyond the horizon then the receiving antenna must be high enough to receive the line-of-sight waves.

The maximum line-of-sight distance dM between the two antennas having heights hT and hR above the earth is given by
d_m = \(\sqrt{2 R h_{T}}+\sqrt{2 R h_{R}}\)
Note:
At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths. At these frequencies, the antennas are relatively smaller.

Modulation And Its Necessity
1. Size of the antenna or aerial:
For transmitting and receiving signal we need antenna having a size comparable to the wavelength of the signal (should have length at least one quarter of the wavelength).

Therefore, to transmit a 1 KHz signal it requires about 500m long antenna, which is practically impossible. This demand that the audio signal is to be converted into a high frequency signal fortransmission.

2. Effective power radiated by an antenna:
To send signals to large distances the power of the transmitter should be as high as possible. Transmission power of an antenna is inversely proportional to the square of the wavelength (P α (l/λ)²). Therefore, to attain high radiation power the wavelength should be as small as possible.

3. Mixing up of signals from different transmitters:
Suppose many people are talking at the same time and those audio signals are transmitting simultaneously. All those signals will get mixed up and there is no way to distinguish between them. This problem can be solved by transmitting the audio signals in the form of high frequency signals.

Modulation:
To overcome all those difficulties (mentioned above) we make use of the technique called modulation. Modulation is the process of super posing a low frequency (audio signal) information on to a high frequency carrier wave.

Carrierwave:
The carrierwave may be sinusoidal wave ora pulse train.

Sinusoidal carrier wave can be mathematically expressed
c(t) = A_c sin (ω_ct + Φ)
where c(t)is the signal strength (voltage or current). A_c is the amplitude, ω_c (2πv_c) is the angular frequency and Φ is the initial phase of the carrier wave.

While modulating, any one of the parameters is varied according to the base band signal (audiosignal). These result in three types of modulation using sinusoidal carrier waves namely

• Amplitude modulation
• Frequency modulation
• Phase modulation

In a similar way, a pulse train is characterized by pulse amplitude, pulse duration or pulse width and pulse position denoted by the rise and falls of the pulse. Hence different types of pulse modulation are

• Pulse Amplitude Modulation (PAM)
• Pulse Width Modulation (PWM)
• Pulse Position Modulation (PPM)

Amplitude Modulation
In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal.

Mathematical analysis:
Consider a sinusoidal carrier wave c(t)=A_c sinω_ct and a modulating signal (message signal) m(t) = A_m sinω_mt.

The message signal is added in such a way to change the amplitude of carrier wave. Hence the modulated signal can be written as,
c_m(t) = (A_c + A_m sinω_m t) sinω_ct
= A_c sinω_c t + A_m sinω_m t sinω_c t
= A_c sinω_c t + µ A_c sinω_c t sinω_m t
where

called modulation index.
Using trigonometric relation sinAsinB = 1/2cos(A – B) – cos(A + B) we can write

The above equation shows that, the modulated signal consists of three frequencies, ω_c, (ω_c – ω_m), (ω_c + ω_m) where (ω_c – ω_m ) is called lower side band frequency and (ω_c + ω_m) is called upper side band frequency.
A plot of A_c with ω for AM signal:

Note:
Modulation index (µ) is always kept ≤1 to avoid distortion.

Production Of Amplitude Modulated Wave
Production of an amplitude-modulated wave is given in a block diagram.

Step – I:
The modulating signal A_msinω_mt is added to the carrier signal A_csinω_ct to produce x(t).
x(t)=A_m sinω_m t + A_c sinω_c t……..(1)

Step – II:
This signal x(t) = A_msinω_mt + A_csinω_ct is passed through a square law device which is a non-linear device which produces an output.
y(t) = B x(t) + C x(t)²………..(2)
where B and C are constants.
Substitute the eq(1) in eq.(2).
y(t) = B [A_m sinω_mt + A_csinω_ct] + C [A_m sinω_mt + A_c sinω_ct]²
y(t) = B A_m sinω_mt + B A_csinω_c + C [A²_m sin²ω_m t + A²_c sin²ω_ct + 2A_mA_c sinω_ct sinω_mt]

Step – III:
The output from the square law device y(t) is passed to Band pass filter. The Band pass filter remove dc component \(\frac{c}{2}\)(A²_m + A²_c) and ω_m, 2ω_m, and 2ω_c from the signal y(t).

Hence the output of band bass filter will be amplitude modulated wave containing three frequencies ω_c, (ω_c – ω_m) and (ω_c + ω_m). ie. Output of band pass filter
= BAω_c sinω_c t + C A_mA_c (cos(ω_c – ω_m )t) + A_mA_c(cos(ω_c + ω_m)t)
The output contain three frequencies ω_c, (ω_c – ω_m) and (ω_c + ω_m).

Transmission of AM wave:

The AM is given to a power amplifier. The power amplifier provides the necessary power and then the modulated signal is fed to an antenna for radiation.

Detection Of Amplitude Modulated Wave
The block diagram of AM receiver is shown in figure.

Step I:
The AM wave is received by the Receiving antenna.

Step II:
The signal from the antenna is given to the amplifier. The amplifier will give sufficient strength to the receiving signal.

Step III:
The output from the amplifier is given to the IF (intermediate frequency) stage. In IF stage, the carrier frequency is changed into a lower frequency.

Step IV:
Detection:
The output from the IF stage is given to the detector. Detection is the process of recovering the modulating signal from the modulated carrier wave. The process of detection is shown in block diagram.

The modulated signal fig (a) is given to the rectifier. The rectifier removes the negative part of the A.M and gives the output as shown in figure (b). This output is given to the envelop detector. The envelop detector gives an output of message signal as shown in figure (c).

Step V:
The message signal from the detector is given to the amplifier. The amplifier, amplifies the signal and given to the loud speaker.

Students can Download Chapter 16 Chemistry in Everyday Life Notes, Plus Two Chemistry Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Chemistry Notes Chapter 16 Chemistry in Everyday Life

Drugs and their Classifcation
Drugs- chemicals of low molecular masses which interact with macromolecular targets and produce a biological response. When this response is therapeutic and useful, these chemicals are called medicines. Medicines are used in diagnosis, prevention, and treatment of diseases. Chemotherapy – Use of chemicals for therapeutic effect.

1. Classification of Drugs:
(a) On the Basis ofPharmacobgical Effect:
Provides them the whole reange of drugs available forthe treatment of a particular type of problem, useful to doctors, e.g. Analgesics – pain killing effect, Antiseptics – kill or arrest the growth of microorganisms.

(b) On the Basis of Drug Action:
Based on the action of a drug on a particular biochemical process, e.g. Antihistamines – inhibit the action of histamine.

(c) On the Basis of Chemical Structure:
Based on the chemical structure of the drug. These drugs share common structural features and often have similar pharmacological activity, e.g. Sulphonamides

(d) On the Basis of Molecular Targets:
Based on molecular targets, most useful for medicinal chemists. Drugs usually interact with biomolecules called target molecules or drug targets.

Drug-Target Interaction
Enzymes and receptros are important drug targets. Enzymes are proteins which perform the role of biological catalysts in the body. Receptors are proteins which are crucial to communication system in the body.

1. Enzymes as Drug Targets:
(a) Catalytic Action of Enzymes:
In their catalytic activity enzymes perform two major functions:

• To hold the substrate molecule in a suitable position, so that it can be attacked by the reagent effectively.
• To provide functional groups that will attack the substrate and carry out chemical reaction.

(b) Drug-Enzyme Interaction:
Drugs can block the binding site of the enzymes and prevent the binding of substrate, or can inhibit the catalytic activity of the enzyme. Such drugs are called enzyme inhibitors. This can be done in two different ways:

• Competitive Inhibitors: drugs which compete with the natural substrate for their attachment on the active sites of enzymes.
•  Allosteric Site: sites other than the active site of the enzyme. Some drugs bind to the allosteric site of the enzyme which changes the shape of the active site in such a way that substrate cannot recognise it.

Receptors as Drug Targets:
Majority of the receptor proteins are embedded in the cell membrane in such a way that their small part possessing active site projects out of the surface of the membrane and opens on the outside region of the cell membrane.

Chemical messengers – Chemicals involved in the transmission of message between two neurons and that between neurons to muscles. These are received at the binding site of the receptor proteins. To accommodate a messenger, shape of the receptor site changes. This brings about the transfer of message into the cell.

Antagonists – drugs that bind to the receptor site and inhibit its natural function. These are useful when blocking of a message is required.

Agonists – drugs that mimic the natural messenger by switching on the receptor. These are useful when there is lack of natural chemical messenger.

Therapeutic Action of Different Classes of Drugs
1. Antacids:
Drugs which control acidity in stomach. In early times, antacids such as NaHCO₃ or mixture of Al(OH)₃ and Mg(OH)₂ were used.

Histamine, stimulates the secretion of pepsin and hydrochloric acid in the stomach. Its discovery helped in the treatment of hyperacidity. The drug cimetidine (Tegamet) prevents the interaction of histamine with the receptors present in the stomach wall.

This resulted in release of lesser amount of acid. This drug was in use until another drug ranitidine (Zantac) was discovered.

2. Antihistamines:
Drugs which act against histamines, a potent vasodilator. It contracts the smooth muscles in the bronchi and gut and relaxes other muscles in the walls of fine blood vessels. It is also responsible for the nasal congestion associated with common cold and allergic response to pollen, e.g. Brompheniramine (Dimetapp), Terfenadine (Seldane) act as antihistamines.

3. Neurologically Active Drugs:
(a) Tranaquilizers:
Chemical compounds used for the treatment of stress and mild or even severe mental diseases. These relieve anxiety, stress, excitement by inducing a sense of well-being, e.g. Chlorodiazepoxide and meprobamate (mild tranquilizers suitable for relieving tension).

Antidepressant drugs – Drugs that inhibit the enzymes which catalyse the degradation of the important neurotransmitter, noradrenaline. Thus, noradrenaline is slowly metabolised and can activate its receptor for longer periods of time, thus counteracting the effect of depression.
e.g. Iproniazid, Phenelzine. Equanikised in controlling depression and hypertension.

Barbiturates – important class of tranquilizers which are derivatives of barbituric acid. These are hypnotic (sleep producing agents), e.g. veronal, amytal, nembutal, luminal and seconal.

(b) Analgesics:
Reduce or abolish pain without causing impairment of consciousness, mental confusion, incoordination or paralysis or some other disturbances of nervous system. They are two types,

(i) Non-narcotic Analgesics:
Drugs effective in relieving skeletal pain such as due to arthritis, also reduce fever (antipyretic), e.g. Aspirin, paracetamol. Aspirin is also used in the prevention of heart attacks because of its anti blood clotting action.

(ii) Narcotic Analgesic:
Drugs which releive pain and produce sleep in medicinal doses. In poisonous doses they produce stupor, coma, convulsions and ultimately death.They cause addiction, e.g. Morphine, Heroin, Codeine.

4. Antimicrobials:
Destroy/prevent development or inhibit the pathogenic action of microbes such as bacteria, fungi, virus or other parasites selectively. Antibiotics, antieptics and disinfectants are antimicrobial drugs.

(a) Antibiotics:
Chemical substances which are produced by microorganism/partly by chemical synthesis and are capable of destroying or inhibiting other micro organisms.
e.g. Salvarsan – Used in the treatment of syphilis. Antibiotics have either cidal(killing) effect ora static (inhibitory) effect on microbes.

Bactericidal	Bacteriostatic
Pecilline	Erythromycin
Aminoglycosides	Tetracycline
Ofloxacin	Chloramphanicol

Antibiotics are also classified into broad spectrum and narrow spectrum antibiotics based on their spectrum of action (range of bacteria or other microorganism that are affected by a certain antibiotic).

(1) Broad Spectrum Antibiotics:
Antibiotics which attack a wide range of Gram-positive and Gram-negative bacteria, e.g. Tetracyclin, steptomycin, chloramphenicol, vancomycin, ofloxacin etc. Ampicillin and Amoxycillin are synthetic modifications of pencillins. These have broad spectrum.

(2) Narrow Spectrum Antibiotics:
Effective mainly against Gram-positive or Gram-negative bacteria, e.g. Pencilline.

(3) Limited Spectrum Antibiotics: Antibiotics effective against a single organism or disease.
Dysidazirine – Antibiotic which is toxic towards certain strains of cancer cells.

(b) Antiseptic and Disinfectants:
Chemicals which either kill or prevent the growth of microorganisms. Antiseptics are applied to the living tissues such as wounds, cuts, ulcers and skin deseases. These are not injected like antibiotics, e.g. Furacine, soframidne.

• Dettol- Commonly used antiseptic, it is a mixture of chloroxylenol and terpineol.
• Bithional – added to soaps to impart antiseptic properties.
• Tincture iodine – 2 – 3% solution of iodine in alcohol-water mixture.
• Dilute aq. solution of boric add- weak antiseptic for eyes.
• Iodoform – antiseptic for wounds.

Disinfectants are applied to inanimate objects such as floors, drainage system, instruments, etc.
e.g. 1% of phenol, SO₂ (in very low concentration) and Cl₂ (0.2 to 0.4 ppm) are common disinfectants. Some substance can act as an antiseptic as well as disinfectant by varying concentration.
e.g. 0.2% phenol – antiseptic
1 % phenol – disinfectant.

5. Antifertility Drugs:
Drugs used to control population. Birth control pills essentially contain a mixture of synthetic estrogen and progesterone derivatives. Progesterone suppresses ovulation. Synthetic progesterone derivatives are more potent than progesterone. e.g. Norethindrone.
Ethynylestradbl(novestrol) – eastrogen derivative used in combination with progesterone derivative.

Chemicals in Food
Chemical are added to food for preservation, enhancing their appeal and adding nutritive value.

1. Artificial Sweetening Agents:
Chemical substances which give sweetening effect to food. Commonly used artificial sweetening agents: Aspartame – 100 times sweeter than cane sugar, methyl ester of dipeptide formed from aspartic acid and phenylalanine. Use of aspartame is limited to cold foods and soft drinks because it is unsatble at cooking temperature.

Saccharine – 550 times sweeter than cane sugar. It is the first popular artificial sweetening agent. It is excreted from the body in urine unchanged. It appears to be entirely inert and harmless when taken. Its use is of great value to diabetic persons and people who need to control in take of calories.

Sucrlose – 600 times sweeter than cane sugar. It is the trichloro derivative of sucrose. Its appearance and taste are like sugar. It is stable at cooking temperature. It does not provide calories.

Alitame – 2000 times sweeter than cane sugar. It is high potency sweetner. The control of sweetness of food is difficult while using it.

2. Food Preservatives:
They prevent spoilage of food due to microbial growth, e.g. Sugar, table salt, vegetable oils, sodium benzoate (C₆H₅ COONa), salts of sorbic acid, and propanoic acid.

Cleansing agents
Detergents and soaps are commonly used as cleansing agents.

1. Soaps:
Sodium/pottassium salts of higher fatty acids or long chain fatty acids like palmitic acid, stearic acid, oleic acid etc. Glyceryl ester of fatty acids is treated with aqueous NaOH solution. This reaction is known as saponification.

In this reaction, esters of fatty acids are hydrolysed and the soap obtained remains in colloidal form. Soap is precipitated from the solution by adding NaCI. Pottassium soaps are soft to the skin than sodium soaps. These can be prepared by using KOH solution in place of NaOH.

Types of Soaps
Toilet soap:
Prepared by using better grade of fats and oils with suitable soluble hydroxide. Colour and perfumes are added to make them more attractive.

Medicated soaps:
These contain substances of medicinal value. In some soaps deodorants are added, e.g. Shaving soaps contain glycerol to prevent rapid drying. A gum called rosin is added which forms sodium rosinate and lathers well.

Laundry soaps:
These contain fillers like sodium rosinate, sodium silicate, borax and soium carbonate. Hard water contains calcium and magnesium ions which form insoluble calcium and magnesium soaps respectively when sodium or potassium soaps are dissolved in hand water.

These insoluble soaps separate as scum in water and are useless as cleansing agent. This precipitate adheres onto the fibre of the cloth as gummy mass and hinders good washing.

2. Synthetic Detergents:
Cleaning agents which have all properties of soaps but actually do not contain any soap. These can be used both in soft and hard water. They are mainly classified into 3 categories.

• Anionic detergents: They are sodium salts of sulphonated long chain alcohols or hydrocarbons, e.g. Sodium dodecylbenzene sulphonate.
• Cationic detergents: They are quarternary ammonium salts of amines with acetate, chlorides or bromides as anions. Cationic part possess a long hydrocarbon chain and a positive charge on nitrogen atom. e.g. Cetyltrimethylammonium bromide.
• Non-ionic detergents: They do not contain any ion in their constitution, e.g. detergent formed by the reaction between stearic acid and polyethylene glycol. Liquid dishwashing detergents are non-ionic type.

The hydrocarbon chain of synthetic detergents is highly branched. Bacteria cannot degrade this easily. Slow degradation of detergents leads to their accumulation. The branching of the hydrocarbon chain is now a days controlled and kept to the minimum. Unbranched chains can be biodegraded more easily and hence pollution is prevented.

Supplementary Material
Antioxidants in Food:
These are important and necessary food additives which help in food preservation by retarding the action of oxygen on food. These are more reactive towards oxygen than the food material which they are protecting, e.g. Butylated Hydroxy Toluene (BHT), Butylated Hydroxy Anisole (BHA).

The addition of BHA to butter increases its shelf life from months to years. Sometimes BHT and BHA along with citric acid are added to produce more effect. Sulphur dioxide and sulphite are useful antioxidants for wine and beer, sugarsyrups and cut, peeled or dried fruits and vegetables.

Students can Download Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits Notes, Plus Two Physics Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus Two Physics Notes Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits

Introduction
Before the discovery of transistor, vacuum tube or valves were considered as the building blocks of electronic circuit.

1. A Comparison of Vacuum Tubes and Transistors:

Vacuum Tubes/valves	Transistors
1. External heating is required. (Electrons are supplied by heated cathode)	No external heating is required.
2. Large evacuated space (vacuum) is required be­ tween cathode and anode	Evacuated space is not required
3. The electrons from heated cathode flows through vacuum.	The charge carriers flows within solid itself.
4. Bulky (large in size)	Small in size
5. Consume high power	Low power consumption
6. Operate at high voltage	Operate at low voltage
7. Limited life arid low reliability.	Long life and high reliability.

Classification Of Metals, Conductors, And Semiconductors
1. On the basis of conductivity:
On the basis of relative values of electrical conductivity (σ) and resistivity ρ = \(\frac{1}{\sigma}\) solids are classified as

(i) Metals:
They possess very low resistivity (or high conductivity).
ρ → 10^-2 – 10^-8 Ω m
σ → 10² – 10⁸ S m^-1

(ii) Semiconductor:
They have resistivity or conductivity intermediate to metals and insulators.
ρ → 10^-5 – 10⁶ Ω m
σ → 10⁵ – 10^-6 S m^-1

(iii) Insulators: They have high resistivity (or low conductivity).
ρ → 10¹¹ – 10¹⁹ Ω m
σ → 10^-11 – 10^-19 S m^-1

2. Band Theory: Conduction Band, Valence Band, and Energy Gap:
In an isolated atom, electrons will have definite energy level. When atoms combine to form solid, the energy levels of outer electrons overlap. Hence outer energy levels split in to many energy levels.

These energy levels are very closely spaced Hence it appears as continuous variation of energy. This collection of energy levels are called energy band.

The energy band which includes energy levels of valence electrons is called valence band. The energy band above valence band which includes energy levels of conduction electrons is called conduction band.

The gap between the top of valence band and bottom of conduction band is called energy band gap (Energy gap, E_g).
Energy level diagram of different bands:

The band gap energy of Ge and Si are 0.3ev and 0.7ev respectively.

3. Classification on the basis of Energy bands Conductors:
Conduction band is partially filled and valence band is partially empty.

OR

Conduction band and valence band are overlapped so that E_g = 0ev

Due to overlapping, electrons are partially filled in conduction band. These partially filled electrons are responsible for current conduction.

Insulators:

Conduction band is empty. Valence band may fully or partially filled. There is a wide energy gap between valence band and conduction band (E_g > 3ev).

Semiconductors:
Conduction band may be empty or lightly filled. Valence band is fully filled. The energy gap is very small (< 3ev)

At room temperature some electrons in valence band get enough energy to cross the energy gap and move into conduction. Hence semiconductors show intermediate conductivity.

Intrinsic Semiconductor
A semiconductor in its pure form is called intrinsic semiconductor.

For intrinsic semiconductor:
* The number of free electrons is equal to number of holes.
ie. n_e = n_h = n_i
n_e, n_h and n_i are the free electron concentration, hole concentration and intrinsic carrier concentration.

Explanation:
Each Si atom is covalently bonded to nearest four neighboring atoms. When temperature increase, some of covalent bond brakes and electrons become free leaving a vacancy (hole). Thus each free electron creates hole in the lattice. Hence number of free electrons equals number of holes.

* The total current in intrinsic semi conductor is the sum of free electron current I_e and hole current I_h.
I = I_e + I_h

Explanation:
When an electric field is applied, free electrons move towards positive potential and give rise to electron current, le. The holes move towards negative potential and give rise to hole current. Thus total current is contributed by both free electrons and holes.

Extrinsic Semiconductor
1. Extrinsic semiconductor or impurity semiconductor:
The addition of suitable impurity improves the conductivity of intrinsic semiconductors. Such semiconductors are called extrinsic semiconductor. They are of two types n-type and p-type semiconductors.

2. Doping and Dopants:
The deliberate addition of suitable impurity to semiconductors to improve its conductivity is called doping.
The impurity atoms are called dopants. There are two types of dopants;

• Pentavalent (valency 5): Arsenic (As), Antimony (Sb), Bismuth (Bi), Phosphorous (P), etc.
• Trivalent (Valency 3): Indium (In), Boron (B), Aluminium (Al), etc.

3. n-type semiconductor:
When a pentavalent impurity is added to Si crystal, four electrons of impurity atom make bond with neighboring four Si atoms. The fifth electron remains weakly bound to its parent atom.

At room temperature this electron become free to move. Thus each pentavalent atom donate one extra electron for conduction and hence it is called donor impurity.

Thus in doped semiconductor the number of conduction electrons will be large compared to number of holes. Hence electrons are the majority carriers and holes the minority carriers. Hence semiconductors doped with pentavalent impurity is called n-type semiconductor.
Note:
In n-type semiconductors
n_e >> n_h
But as a whole n-type semiconductor is neutral (ie. electrons is equal and opposite to ionized (donor) core in lattice).

4. p-type semiconductor:
When a trivalent impurity is added to Si crystal, three electrons of impurity atom make covalent bond with neighboring three Si atom. The fourth bond with neighboring Si atom lacks one electron. Thus a vacancy or a hole is created in fourth bond.

The neighboring Si atom needs an electron to fill the vacancy and hence one electron in outer orbit of nearby Si atom move to this vacancy leaving a hole in its own site. Thus hole can move through the lattice.

Each trivalent atom creates a hole and it act as acceptor. Hence it is called acceptor impurity. The semiconductor doped with trivalent impurity has more number of holes than free electrons. Here holes are the majority carriers and electrons are the minority carriers. Hence it is called p-type semiconductor.
Note: I
(I) In p-type semi conductor
n_h >> n_e
But as a whole p-type semiconductor is electrically neutral. (The charge of additional holes is equal and opposite to acceptor ions).

(II) In thermal equilibrium electron and hole concentration in a semiconductor is given by n_en_h = n²_r.

5. Energy band structure of Extrinsic semiconductors
n-type semiconductor:

In n-type semiconductors, the donor energy level (E_D) is slightly below conduction band.

P-type semiconductor:

In p-type semiconductors, the acceptor energy level (E_A) lies slightly above valence bond.

p-n Junction
A p-n junction is basic building block of semiconductor devices like diode, transistor, etc.

1. p-n junction formation:
When pentavalent impurity is added to a part of p-type Si semiconductor wafer, we get both p region and n region in a single wafer.
The formation of p-n junction includes two processes.

(i) Diffusion:
In n type semiconductor, concentration of electrons is more than that of holes. In p-region, the hole concentration is more than electron concentration. Because of this concentration gradient, electrons diffuse from n side to p-side and holes diffuse from p-side to n-side during the formation of p-n junction. This produces diffusion current.

(ii) Drifting – Formation of Depletion region:

When electrons diffuses from n to p, it leaves behind positively charged immobile donor ions on n-side. As electrons continue to diffuse from n to p, a layer of positive charge is developed on n- side.

Similarly when holes diffuse from p to n, it leaves behind negatively charged immobile ions on p side. As holes continue to diffuse from p to n, negative space charge region is developed on p side.

The positive space-charge region on n-side and negative space-charge region on p-side, is known as depletion region. This region contain only immobile ions.

2. Barrier Potential:
The n-region losses electrons and p-region gains electrons. Because of this a potential is developed across the junction. This potential is called barrier potential.

Semiconductor Diodes
A semiconductor diode is a p-n junction provided with metallic contact at both ends to apply external voltage.
The symbol of p-n junction diode is given below.

The arrow shows conventional direction of current.

1. p-n junction diode under forward bias:
When p-side of p-n junction diode is connected to positive terminal of the battery and n-side to the negative terminal it is said to be in forward biased.

Due to the applied voltage, electrons of n-side get repelled by negative terminal of battery. Hence they cross depletion region and reach at p-side.

similarly the holes of p-side get repelled by positive terminal of battery and cross depletion region, reach n-side. The total forward current is sum of hole current and current due to electron.

2. p-n junction diode under reverse bias:
When p-side of p-n junction diode is connected to negative terminal of battery and n-side to the positive terminal, it is said to be in reversed biased.

In reverse bias the electrons of n-side and holes on p-side cannot cross the junction. But the minority carriers – holes on n-side and electrons on p-side drift across the junction and produce current. The reverse current is of the order micro ampere.
Note: Junction width increases in reverse bias.

Breakdown Voltage (V_Br):
The reverse current remains independent of bias voltage up to a critical reverse bias voltage called reverse break down voltage. At breakdown voltage, reverse current increases sharply.

V-I characteristics:
To study variation of current with voltage for p-n junction diode, it is connected to a battery through a rheostat. Rheostat is used to vary the biasing voltage. A milliammeter is connected in series with diode to study forward current.

To measure reverse current micro ammeter is used. A voltmeter is connected across diode to measure voltage. The current is measured for different values of volt and a graph (V-I) is plotted.

In forward bias, current first increases very slowly up to a certain value of bias voltage. After this voltage, diode current increases rapidly. This voltage is called Knee voltage or cut-in voltage or threshold voltage. (0.2v for Ge and 0.7v for Si). The diode offers low resistance in forward bias.

In reverse bias, current is very small. It remains almost constant up to break down voltage (called reverse saturation current). Afterthis voltage reverse current increases sharply.
Note:
(i) In forward bias, resistance is low compared to reverse bias.
(ii) The dynamic resistance of diode is defined as ratio of change in voltage to change in current.
r_d = \(\frac{\Delta v}{\Delta l}\)

Application Of Junction Diode
Diode as a rectifier:
The process of converting AC into DC is known as Rectification. A p-n junction diode conducts current when it is forward biased, and does not conduct when it is reverse biased. This feature of the junction diode enables it to be used as rectifier.

1. Diode as half wave rectifier:

Circuit details:
A half wave rectifier consists of transformer, a diode and a load resistor R_L. The primary coil of transformer is connected to a.c input and secondary is connected to R_L through diode.

Working:
During the positive half cycle of the input a.c at secondary, the diode is forward biased and hence it conducts through R_L. During negative half cycle of a.c at secondary, diode is reverse biased and does not conduct. Thus, we get +ve half cycle at the output. Hence the a.c input is converted into d.c output.

2. Full wave rectifier:
Circuit details:

Full wave rectifier consists of transformer, two diodes, and a load resistance R_L. Input a.c signal is applied across the primary of the transformer. Secondary of the transformer is connected to D₁ and D₂. The output is taken across R_L.

Working:
During the +ve half cycle of the a.c signal at secondary, the diode D₁ is forward biased and D₂ is reverse biased. So that current flows through D₁ and R_L.

During the negative half cycle of the a.c signal at secondary, the diode D₁ is reverse biased and D₂ is forward biased. So that current flows through D₂ and R_L.

Thus during both the half cycles, the current flows through R_L in the same direction. Thus we get a +ve voltage across R_L for +ve and -ve input. This process is called full wave rectification.

Special Purpose p-n Junction Diodes
1. Zener diode:
Zener diode is designed to operate under reverse bias in the breakdown region. It is used as a voltage regulator. The symbol for Zener diode is shown in figure.

Zener diode is heavily dopped. Hence depletion region is very thin.
I-V characteristics of zener diode:

The l-V characteristics of a Zener diode is shown in above figure. At break down voltage, current increases rapidly. After breakdown, zener voltage remains constant. This property of the Zenerdiode is used for regulating supply voltages.

Explanation for large reverse current:
Reverse current is due to the flow of electrons (minority carriers) from p to n and holes from n to p. When the reverse bias voltage increases and becomes V = V₂ high electric field is developed. This high electric field can pull valence electrons from the atoms. These electrons account for high current.

1. (a) Zener diode as avoltage regulator Principle:
In reverse breakdown region, the voltage across the diode remains constant.
Circuit details:

The zenerdiode is connected to a fluctuating voltage supply through a resistor R_z. The out put is taken across R_L.

Working:
When ever the supply voltage increases beyond the breakdown voltage ,the current through zener increases (and also through R_z).

Thus the voltage across R_z increases, by keeping the voltage drop across zenerdiode as a constant value. (This voltage drop across R_z is proportional to the input voltage).

2. Optoelectronic junction devices:
(i) Photodiode:
The photodiode can be used as a photodetector to detect optical signals.

It is operated under reverse bias. When the photodiode is illuminated with light (photons) electron-hole pairs are generated. Due to electric field of the junction, electrons and holes are separated before they recombine.

The direction of the electric field is such that electrons reach n-side and holes reach p-side. Electrons collected on n-side and holes collected on p- side produce an emf. When an external load is connected, the current flows through the load.
The I-V characteristics of a photodiode:

(ii) Light emitting diode [LED]:
LED is heavily doped pn junction diode working under forward bias .Gallium Arsenide is used for making infrared LEDs.

Working:
When the junction diode is forward biased, electrons and holes flow in opposite directions across junction. Some of the electrons and holes combine at junction and energy is produced in the form of light.

Uses:
LEDs are used in remote controls, burglar alarm systems, optical communication, etc.

Advantages of LED over conventional incandescent lamps:

1. Low operational voltage and less power.
2. Fast action and no warm-up time required.
3. The bandwidth of emitted light is 100 A° to 500 A° or in other words it is nearly (but not exactly) monochromatic.
4. Long life and ruggedness.
5. Fast on-off switching capability.

3. Solar cell:
Solar cell is junction diode used to convert solar energy into electrical energy.
Circuit details:

Its p-region is thin and transparent and is called emitter. The n-region is thick and is called base. Output is taken across R_L.

Working:
When light falls on this layer, electrons from the n-region cross to the p-region and holes in the p-region cross in to the n-region. Thus a voltage is developed across R_L. Solar cells are used to charge storage batteries during daytime.
The I-V characteristics of a solar cell:

The I-V characteristics of solar cell is drawn in the fourth quadrant of the coordinate axes. This is because a solar cell does not draw current but supplies the same to the load.

Junction Transistor
1. Transistor: structure and action
Transistor is a three layered doped semiconductor device. There are two types of transistors:

• n-p-n transistor
• p-n-p transistor.

Symbols:

Naming the transistor terminals:
A transistor has 3 terminals:

1. Emitter
2. Collector
3. Base.

1. Emitter:
The section, which supplies charge carriers, is called emitter. Emitter is heavily doped. The emitter should be forward biased.

2. Collector:
The section which collects the charge carriers, is called collector. Collector is moderately doped. The collector should be reverse biased.

3. Base:
Middle section between emitter and ‘ collector is called base. Base is lightly doped.

Transistor action:
Circuit details:
Emitter is maintained at forward bias and collector is maintained at reverse bias. V_EB is the emitter base voltage and V_CB is the collector base voltage.

Working:
Emitter is kept at forward bias so that the electrons are ejected into base. Thus an emitter current I_E is produced.

At the base, electron hole combination takes place. As the base is lightly doped and very thin, only a few electrons combine with holes to constitute the base current, I_B.

The remaining electrons are attracted towards collector because the collector is kept at reverse bias. Due to this electron flow, a collector current I_C is produced.

In this way, the emitter current is divided into base current and collector current.
Mathematically this can be written as
I_E = I_B + I_C
I_B > is small, so I_E = I_C

2. Basic transistor circuit configurations and transistor characteristics:
Transistor can be used in three modes:

• Common base configuration
• Common emitter configuration
• Common collector configuration

a. Common base configuration:

In Common base configuration .base is common to both input and output
Current amplification = \(\frac{\text { output current }}{\text { input current }}\)
Current amplification, α = \(\frac{l_{C}}{l_{E}}\)

b. Common emitter configuration:

Current amplification = \(\frac{\text { output current }}{\text { input current }}\)
Current amplification, β = \(\frac{l_{C}}{l_{B}}\)

c. Common collector configuration:
Current amplification γ = \(\frac{l_{E}}{l_{B}}\)

Relation between α and β:
i.e. β = \(\frac{\alpha}{I-\alpha}\)
Common Emitter Configuration:

Input Characteristics (CE configuration):
The graph connecting base current with base emitter voltage (at constant V_CE) is the input characteristics of the transistor.

To study the input characteristics, the collector to emitter voltage (V_CE) is kept at constant. The base current I_B against V_BE is plotted in a graph.
The ratio ∆ V_BE/∆ I_B at constant VCE is called the input resistance.
i.e,,Input resistance \(r_{1}=\frac{\Delta V_{B E}}{\Delta I_{B}}\)

Output Characteristics (CE. Configuration):
The output characteristics is a graph connecting the collector current lc with collector-emitter voltage (V_CE) at a constant base current (I_B).

This is obtained by measuring the collector current I_B at different collector voltage by keeping the base current fixed.

Line OA is called saturation line .The region right of the saturation line is the active region. Transistor is operated as amplifier in this region. The region below I_B = 0 is the cut off region.

The output resistance is the ratio of a small change in collector voltage to the change in collector current at constant base current.
Output resistance \(\mathrm{r}_{0}=\frac{\Delta \mathrm{V}_{\mathrm{CE}}}{\Delta \mathrm{I}_{\mathrm{C}}}\)

3. Transistor as a device
Transistor as a switch:
A circuit diagram for transistor switch is given below.

Applying Kirchoff’s voltage rule to the input side of this circuit, we get
V_BB = I_BR_B + V_BE
and applying Kirchoff’s voltage rule to the output side of this circuit, we get
V_CE = V_CC – I_CR_C.
We shall treat V_BB as the dc input voltage V_i and V_CE as the dc output voltage V_o.
So, we have
V_i = I_BR_B + V_BE ____(1) and
V_o = V_CC – I_CR_i______(2)

The variation of output voltage with input voltage:

The variation of output voltage with input voltage is shown in the above graph. This graph contain three regions

• cut off region
• Active region
• saturation region.

a. Cut off region:
In the case of Si transistor, if input voltage V_i is less than 0.6V, the transistor will be in cut off state and out put current (I_c) will be zero.
Substituting I_c = 0 in the eq (2) we get out put voltage V_o = V_CC

b. Active region:
When V_i becomes greater than 0.6 V the transistor is in active state with some current I_c. The eq(2) shows that, the output V_o decrease as the term I_cR_c increases. With increase of V_i, I_c increases almost linearly and so V_o decreases linearly till its value becomes less than about 1.0 V.
Note:
Amplifier is working in the active region.

c. saturation region:
When the output voltage becomes 1.0V, the change becomes non linear and transistor goes into saturation state. With further increase in V_i the output voltage is found to decrease towards zero (though it may never become zero).

Working of transistor as switch:
When V_i is low (unable to give forward-bias to the transistor) we get high output (ie. V_o = V_cc). In this stage the transistor doesn’t conduct. Hence transistor is said to be switched off.

If V_i is high enough to drive the transistor into saturation, then V_o is low (very near to zero). In this stage the transistor driven into saturation it is said to be switched on.
Note:
The switching circuits are designed in such a way that the transistor does not remain in active state.

4. Transistoras an Amplifier (CE-Configuration):

The working of an amplifier can be explained using the circuit given above. It is an n-p-n transistor connected in common emitter configuration. V_BB is the biasing voltage used in the input side and V_cc is the reverse bias voltage used in the output side.

R_B is the resistor connected to base in order to reduce the base current. R_c is the resistor which is connected in between V_cc and collector terminal. We take the voltage across R_c and V_cc with the help of a capacitor C. We maintain voltages V_BB and V_cc such that the transistor is always on the active region.

Working:
Case 1:
When there is no input signal (ie. V_i = 0,)
The input voltage can be written as
V_BB = V_BE + I_BR_B
This base voltage produces a base current I_B which in turn produces a dc collector current I_C. The output voltage can be written as
V_CE = V_CC – I_CR_C
This dc output voltage is unable to produce an output signal due to the presence of a capacitor. Because, the capacitor prevents the flow of dc current through it.

Case 2:
When there is an input ac signal, (ie. V_i ≠ 0):
when we apply an AC signal as input, we get an AC base current denoted by i_B. Hence input AC voltage can be written as
V_i = i_Br ______(1)
where ‘r’ is the effective input resistance.
This AC input current produces an AC output current (i_c) which can flow through a capacitor. Hence the output voltage can be written as
V₀ = i_c × output resistance
If we take output resistance as R_L then v_o becomes
V₀ = i_c R_L
V₀ = β_AC i_c × R_L _____(2) [since β_AC = \(\frac{\mathrm{i}_{\mathrm{C}}}{\mathrm{i}_{\mathrm{B}}}\)]

Power gain:
The power gain Apcan be expressed as the product of the current gain and voltage gain.
ie. power gain A_ρ = β_ac × A_v
Note:
The transistor is not a power generating device. The energy for the higher ac power at the output is supplied by the battery.

5. Feedback amplifier and transistor oscillator 9.13 Oscillator:
Atransistor amplifier can be converted in to oscillator by positive feed back, (positive feed back means that, a small portion of the out put signal is applied to the input in phase).

Circuit Details:
The battery V_cc is connected in between C (collector) and E (emitter) through a coil L¹. Another coil Lis connected in between B (base) and E. A capacitor is connected in parallel to coil L.

Working:


When the key is pressed ,a small current flows through the coil L¹1. The variation of current in the coil L¹ produces a change in flux. This change in flux induces a voltage across L.

As a result, the forward voltage increases which further increases the emitter and collector current. This again increases the forward voltage. This process continues till the collector gets saturated.

When the collector current is saturated (constant), the flux also become steady and the induced emf becomes zero. This reduces collector current. The decrease in collector current induces a voltage in L in the opposite direction (reverse voltage). As a result the collector current decreases further.

This continues until the collector current falls below its normal value. After this, the collector current build up and the process is repeated. Thus oscillation of frequency.
f = \(\frac{1}{2 \pi \sqrt{L C}}\) is produced.

Digital Electronics And Logic Gates
In digital electronics we use two levels of voltage (represented by 0 and 1). Such signals are called digital signals. Logic gates are the building blocks of digital circuits. Logic gates are used in calculators, digital watches, computers, robots, industrial control systems, and in telecommunication.

1. Logic gates:
A logic gate is a digital circuit that follows certain logical relationship between input and output voltage. Hence it is so called. The funda¬mental logic gates are AND, OR, NOT, NAND, and NOR. The truth table gives all possible input logic level combinations with their respective output logic levels.

(i) NOT gate:
The most basic gate which has only a single input and single output. It is also called inverter. It produces an inverted version of input. The Boolean expression is y = \(\overline{\mathrm{A}}\)
The symbol is

A The truth table is

Input	Output
A	Y
1	0
0	1

(ii) OR gate:
It has two or more inputs but a single output. The output is high when either inputs or both inputs are high.
The Boolean expression is Y = A + B (read as A or B)
The symbol is

The truth table:

(iii) AND gate:
It has two or more inputs but a single output. The output is high only if both inputs are high. The Boolean express of output is Y = A.B (read as A and B)
The symbol is

The truth table

(iv) NAND gate (or bubbled AND gate):
This is an AND gate followed by NOT gate. The Boolean expression is y = \(\overline{\mathrm{A.B}}\)
The symbol is

The truth table

(v) NOR gate (or bubbled OR gate):
It has two or more inputs and one output. This is OR gate followed by NOT gate.
The Boolean expression is Y = \(\overline{A+B}\)
The symbol is

The truth table is

NAND gate and NOR gate are called universal gates because other basic gates like OR, AND and NOT gate can be realized using them.

Integrated Circuits
The entire circuit fabricated on a small piece of semiconductor or chip is called Integrated Circuit (IC). It contain many transistors, diodes, resistors, capacitors, connecting wires – all in one package.

It was invented by Jack Kilky in 1958 and won Nobel prize for this invention. IC’s are produced by a process called photolithography. IC’s are categorized depending on nature of input signals.

(a) Linear or analogue IC:
These IC’s handle analogue signals and output varies linearly with input.
Eg: Operational Amplifier

(b) The digital IC:
These type handles digital signals and mainly contain logic gates Depending on the level of integration (number of circuit components or logic gates), IC are classified as

• SSI – Small scale Integration (logic gates ≤ 10)
• MSI – Medium Scale Integration (logic gates ≤ 100))
• LSI – Large scale Integration (logic gates ≤ 1000)
• VLSI – Very Large scale integration (logic gates > 1000)

The miniaturization in electronics technology is brought about by the Integrated circuit. It has made the things faster and smaller. IC is the heart of computer system. In fact IC’s are found in almost all electrical devices like cars, televisions, CD players, cell phones, etc.