# Kerala Syllabus 8th Standard Basic Science Solutions Chapter 20 Static Electricity

## Kerala State Syllabus 8th Standard Basic Science Solutions Chapter 20 Static Electricity

When some substances are rubbed against each other they can attract other objects. When the substances are rubbed the electrons are exchanged. An atom of a substance gets positive charge on losing electron and negative charge on receiving electron.

Proton, neutron, and electron are the subatomic particles in an atom. Neutron does not possess any charge. Proton have positive charge and electron have negative charge. In any atoms, the number of elect¬ron and proton are equal. So an atom is electrically neutral.

Electrification or charging is the process of converting an object in to electrically charge One. If the electric charge produced in an object remains at the same place in it, it is called static electricity. When a glass rod is rubbed on silk, electrons are lost from the glass rod. Then the glass rod gets positive and silk gets negative charged.

When an ebonite rod is rubbed with wool, ebonite rod gets negative and wool gets positive charged. The metals also charged when rubbed but the charge will spread overall part of the metallic substance. This causes current electricity, there is no static electricity in metals.

Properties of electric charges

• Charged bodies attract uncharged bodies.
• Unlike charges attracts each other
• Like charges repel each other As the charged bodies attract uncharged bodies if two bodies attract each other we cannot say certainly both the bodies possess charge. But if two bodies repel each other it is sure that both bodies possess like charge.

The unit of charge is coulomb. Charge is a scalar quantity. The electroscope is the instrument to know the presence of static electric charge. It is an instrument in which a metallic conductor, gold leaves and a metal ball are connected in a glass jar.

The activity to neutralize the charge on a body is known as discharging.

Earthing

Connecting a body to the earth using metallic conductor is known as earthing. On earthing a charged body, electrons pass from the earth to the body or from the body to the earth to neutralize the charge on the body completely. The give or receives electrons at any time and any quantity. So earth is referred to as an electron bank. If a positively charged body is earthed the electrons flow from earth to the body and if it is charged negatively, the electrons flow to the earth.

Electrostatic induction

On bringing a charged body near to an object the charge is induced to that body. The redistribution of charges in a body as a result of the presence of another charged body is called electrostatic induction. Due to the presence of charged body, the opposite charge is induced on the other body. Then they are attracted each other. If the bodies are in contact due to attraction the opposite charges are canceled.

Capacitor

When a metallic plate is placed near another plate which is positively charged, there is an electric field is formed between them. With this arrangement, electric charge can be stored and retained for a long time. The device which can store electric charge in this way is a called a capacitor. The charge is distributed in a conductor would be only on its surface. The amount of charge would be more at the pointed ends.

Lightning

Lightning is the electric discharge between charged clouds in the atmosphere or between charged clouds and the earth

Lightning conductor

To protect the buildings from lightning, an earthed conductor with pointed end is placed on the highest part of the building. This is lightning conductor.

Methods to protect from lightning

• Do not operate electrical equipment
• Do not lean on the wall of the house.
• Do not stand holding windows bar or grill
• Do not stand beneath tall trees
• Do not take shelter under isolated trees
• Do not stand at open place
• A person struck by lightning should be made to lie in a place where there is good air circulation. In order to prevent blood clotting keep the body warm by massaging. Give artificial respiration and press strongly on the chest to maintain ‘ heartbeat and the person must be taken to a hospital immediately

### Static Electricity Textbook Questions and Answers

Question 1.
From which one to which does electronic transfer occur when the pair of substances given below are rubbed against each other?
a. Glass rod – Silk doth
b. Ebonite – Wool
a. From glass rod to silk
b. From wool to ebonite

Question 2.
Among the substances given below, which cannot be electrified by friction? Why? Amber, Plastic, Hacksaw blade, PVC pipe.
Hacksaw blade. Because it is metal

Question 3.
lightning conductors, which are not properly earthed, produce disaster rather than benefit. Justify this statement.
Even though all the lightning will not affect the building, if there is lightning conductor the opposite charge of that in the clouds is induced on the pointed end of the conductor. If there is no earthing the flow of electron will be from the building to the conductor or to the building which causes accident

Question 4.
What are the precautions to be taken for protecting ourselves from lightning?

• Do not operate electrical equipments
• Do not lean on the wall of the house.
• Do not stand holding windows bar or grill
• Do not stand beneath tall trees
• Do not take shelter under isolated trees

Question 5.
How would you convert a copper plate kept in an electroscope into an arrangement for storing charge for a long time, through induction?
Place copper sheet around the leaves of an electroscope. Touch the nob with a charged, body. Opposite charge is formed on the leaves. Opposite charge of it is formed inside the sheet and the same charge is formed outer surface of the sheet. Now the sheet is earthed, the charge on outer surface will be disappeared.

Question 1.
Complete the table

 Objects Electron exchange Positive Negative Glass, silk From glass rod to silk ….. a ….. Silk Ebonite, wool Wool to ebonite …. b …. …. c …. Rubber rod, wool …. d …. Wool …. e…

a. Glass
b. Wool
c. Ebonite
d. From wool to rubber rod
e. Rubber rod

Question 2.
When a glass rod is rub with silk electron loses from the glass rod. If we bring another glass rod near to it what can be observed? Give reason
They will repel. Same charges repel each other.

Question 3.
Tick the correct one
1. Object with same charge attract/repel
2. The charged body attract/ repel the neutral body
3. bodies with different charges attract/repel.
1. repel
2. attract
3. attract

Question 4.
What is the use of capacitor?
To store the charge

Question 5.
What is earthing?
Connecting an object to earth with a conductor is called earthing.

Question 6.
Examine the following figures and mark the point which charge is accumulated.

Question 7.
Complete the figure.

Question 8.
Why does the pointed edged con-ductors are used as lightning conductor
The charge accumulated at the pointed edge easily and discharge it to earth immediately. So the building is not affected by lightning

Question 9.
What are the precautions to be taken when lightning

• Do not operate electrical equipment
• Do not lay on the wall of the house.
• Do not stand holding windows bar or grill
• Do not stand beneath tall trees
• Do not take shelter under isolated trees

Question 10.
What are the primary steps to be adopted when a person is struck by lightning
A person struck by lightning should be made to lie in a place where there is good air circulation. In order to prevent blood clotting keep the body warm by massaging. Give artificial respiration and press strongly on the chest to maintain heartbeat and the person must be taken to a hospital immediately

Question 11.
What is dielectric
The dielectric are the insulators used in between the plates of the capacitor to increase its capacity Eg. Plastic, polyester, air, paper

Question 12.
What is capacitance? What is its unit
capacitance is the ability to store the charge in it. Its unit is fared(F).

Question 13.
You might have seen iron chains, suspended from the body of lorries and trucks touching the earth. What is this for?
These lorries and trucks have to travel a long distance. Sometimes the friction between rod and tire or vehicle and oil in it produces static electricity. This discharge gathers at one end of the truck and it causes sparking and from it the petrol may catch fire. The iron chains suspended from the body of lorries and trucks provide earthing and help to avoid the hazard.

Question 14.
The installation of lightning conductors is not insisted in may tall modern concrete buildings, nowadays. How are they protected from lightning?
In tall modern buildings, there are plenty of metal rods inside the concrete work, connecting from the top to bottom of the building and to the earth. Electric charge from the lightning clouds, reaching the building, is conducted safely to the earth through these metal rods. So the building are protected from damage due to lightning discharge

Question 15.
A person takes shelter under a tree during heavy rain. Another person shelters under a coconut tree of same height. Who will be safer during lightning? Why?
The person standing under the tree is safer. Electric charge always get concentrated at sharp edges. So more possibility for danger due to lightning is to the man under coconut tree than the. another one

Question 16.
There are two houses nearby, both of them having T.V. antenna pipes. One of these pipes is earthed. Write down the advantages or disadvantages of earthing the pipe.