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Kerala State Syllabus 8th Standard Basic Science Solutions Chapter 11 Magnetism
Nowadays artificial magnets are used commonly. They are made by the alloy like alnico.
A magnet possesses directional property that a freely moving magnet always aligns itself in the north-south direction. If a magnet is arranged in such a way that it can move freely it will align itself in the north-south direction of the earth. It is because a magnet possesses directional property that a freely moving magnet always aligns itself in the north-south direction.
As far as magnets are concerned, like poles repel each other and unlike poles attract each other.
A compass is an instrument which is utilised the directional property of a magnet. Every magnet has two poles even if they are very small.
Earth as a magnet
The earth acts like a large magnet. This was first discovered by the scientist William Gilbert. He found out that just as the earth has north and south geographically, it has the north pole and the south pole when considered as a magnet. The south pole of the earth’s magnet is near the geographic north pole and the north pole of the earth’s magnet is near the geographic south pole.
Magnetic lines of force are experienced in all dimensions around a magnet. The magnetic lines of force is experienced according to the strength of the magnet. Magnetic force is experienced in all dimensions around a magnet. This region around a magnet where the influence is felt is the magnetic field.
The flux density is the number of flux lines passing through unit area.
peculiarities of magnetic lines of force.
- Magnetic lines of force do not intersect one another.
- Magnetic lines of force bend sideways, when like poles of magnets come near each other.
- When unlike poles of magnets come close to each other, the path of the magnetic lines of force is from the north pole of one magnet to the south pole of the other magnet.
The substances which are attracted by magnet is called magnetic substances.
Bring a pin in contact with the pole of a bar magnet. Is it not attracted? What if you bring another pin to the free end of the pin? It will also be attracted. If we bring another pin the second pin will attract the third. But when the first pin is removed from the magnet all pins will fall down. That is the pin acquired the magnetic force from the magnet.
The phenomenon of a magnetic substance acquiring magnetism due to the presence of a magnet is Magnetic Induction. The magnetic force acquired by the magnetic substance is the Induced Magnetism.
Whether under contact or without contact, the polarity of the magnet produced by induction will be like polarity at the further end and unlike polarity at the nearer end.
The ability of magnetic substances to get magnetised under the influence of a magnetic field is Susceptibility. Retentivity is the ability to retain the magnetism thus acquired. Soft iron has high susceptibility. But the ability to retain magnetism acquired (retentivity) is very low.
Permeability is the ability of substances to pass magnetic lines of force through them.
The ability of soft iron to permit magnetic lines of force pass through it is greater than that of air. That is, magnetic lines of force pass more easily through soft iron than through air.
Electromagnets can be made by passing electric current through insulated copper wire wound on a soft iron piece. The strength of electromagnets made in this way depends on
- the number of turns of coiled conductor
- the strength of current
- the area of cross section of the soft iron placed inside the coil
All electromagnets are temporary magnets. If the current ceases to flow, the magnetism of the soft iron core is lost.
Magnetism Textbook Questions and Answers
Basic Science for Class 8 Chapter 11 Question 1.
In the figure, AB is a bar magnet. CD is a soft iron rod placed near the end B. Write down which poles develop at the ends C and
Basic Science Solution Question 2.
Figures A and B show two iron nails each hanging from a bar magnet and a U shaped magnet.
a. Identify the correct picture in A and B.
b. Justify your answers.
a. A – Fig III B – Fig II
b.In A two nails are hang from bar magnet. So the distant ends will be induced the same pole. So repulsion. But in U magnet at the distant opposite poles will be induced. So attraction.
Magnetism Class 8 Notes Question 3.
You are given a soft iron piece, a steel piece of the same size, insulated copper wire and a battery.
a. Explain how a strong permanent magnet can be made.
b. Suggest a method to make a temporary magnet.
a. Take a piece of steel and bind a insulated copper wire. Connect the ends of the wire to a cell
b. Wind a few turns of insulated copper wire on an iron nail. Connect the ends of the wire to a cell
Can you build a compass and make it work by arranging a magnetic needle in such a way that it rotates freely inside an iron case? What is the reason?
No. As the iron is a magnetic substance compass will not rotate freely.
In an exhibition, a plastic car with an iron piece fixed inside it is made to run on a wooden table by sliding a strong magnet below it. The experiment failed when a steel table was used instead. Explain the reason.
Steel table has high permeability. It permit magnetic flux to pass through it. Therefore we cannot slide the car using magnet.
Magnetism Additional Questions and Answers
Alnico Magnet Question 1.
Name the following magnet.
a. Bar magnet,
c. U magnet,
d. Ring magnet,
e. Magnetic plate
Questions on Magnetism for Class 8 Question 2.
Which instrument is used to identify the direction in ship journey in ancient time?
Directional Property of Magnet Question 3.
A bar magnet is hanged by a thread. Where does the north pole of the magnet point?
The north pole of the compass points towards north of the earth that is south pole of earth’s magnet be¬cause earth is a magnet.
Write the magnetic substances from the following.
Wooden block, iron nail, paper, nickel, gold, cobalt, aluminium
Iron nail, nickel, cobalt
Write the right statement
Greater susceptibility (Soft iron/Steel)
Greater retentivity? (Soft iron/ Steel)
a. Soft iron b. Steel
Magnetism Chapter for Class 6 Question 6.
Write three method to increase the strength of an electromagnet.
- the number of turns of coiled con ductor
- the strength of current
- the area of cross section of the soft iron placed inside the coil.
Magnetism Textbook Question 7.
You are given a bar magnet. The poles are not marked on it. Suggest a method to identify the poles of the magnet.
The magnet is suspended using a thread so that it can oscillate freely in a horizontal plane. When it comes to rest the end pointing south can be marked as south pole.
Magnetism Solutions Question 8.
A magnet is dipped in iron filing, the iron fillings are more attracted towards the poles of the magnet than towards other places. Why?
The intensity at the poles is greater than that at other places.
North of geomagnet inclined against 110 with geoscientific north pole? Examine the statement.
The statement is incorrect. North of geomagnet inclined against 110 with geoscienfic south pole. Magnet suspended on geomagnet alinged south – north direction is due to the attraction of unlike poles. North pole (N) of magnet attracted towards the south pole of geomagnet.
What are the difference in the magnetic properties of iron and steel?
Tron becomes a more intense magnet on magnetic induction. But that magnetic power is temporary. When the permanent magnet is removed it losses its magnetic power. In the presence of a magnet, steel doesn’t acquire the magnetic power like that of a soft iron. But steel retains most of its magnetic power after removing the permanent magnet.
Write your inferences.
a. All magnets have same shape.
b. All magnets show N, S direction when suspended freely.
c. All magnets are man made.
d. All magnets show N, S direction when suspended freely.
a. No, There are different types of magnets.
b. Yes, Because the north pole of magnet turns to geographical south of earth and south pole turns to geographical north of earth.
c. No, There are natural and man made magnets.
d. Yes. Earth acts like a large magnet. Hence the poles of the suspended magnet is opposite to the poles of the earth’s magnet.
When an iron nail is wounded with copper wire and connected to a battery, it doesn’t become a magnet, select the reasons from the following.
a. Battery has not enough charge.
b. The insulation at the ends of the wire is not removed.
c. The nail is wounded with non insulated copper wire.
d. The polarity of the battery is changing.
An Iron nail is suspended at the
north pole of a bar magnet. Bring the north pole of another magnet near the free end of a nail. What is observed? What is its reason?
The free end of an iron nail repel. Because the free end of an iron nail becomes north pole by magnetic induction.
Complete the figure
The pole of a magnet attracts an iron nail which has no magnetic power. Describe the work-ing behind this.
The magnet attracts the iron nail brought near the pole of a magnet really as the result of magnetic induction. If the magnetic material reaches near the magnetic field of the magnet, an unlike pole is induced at its near end. The pole of permanent magnet is attract the newly formed unlike poles. That is why the magnetic pole attracts a magnetic material near it.
What are the factors that the attractive force of two magnetic poles depend on?
The attractive force between the magnetic poles increases with increase in the strength of the pole and decreases with increase in the distance between the poles.
What is meant by atomic mag-nets?
Each atom of a magnetic substance is an independent magnet. They are called atomic magnets.
What is meant by line of force (line of flux)?
The line of force is the path that the north pole of a free magnetic needle would take around the bar magnet.
Examine the figure and mark the poles