Class 8 Basic Science (English Medium) Chapter 11 Magnetism - Questions and Answers
Textbooks Solution for Class 8th Basic Science (English Medium) കാന്തീയത | Text Books Solution Physics (English Medium) Chapter 11 Magnetism
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SCERT Solutions for Std VIII Basic Science Chapterwise
Class 8 Physics - Questions and Answers
Chapter: 11 - Magnetism
1. Artificial magnets are used for practical purposes.
a. What are the limitations of natural magnets?
b. Suggest the name of a substance for making an artificial magnet.
c. Give the name of a device that makes use of the directional property of the magnet.
Ans.
a. Natural magnets do not have the required shape and sufficient strength. b.Alnico.
c. Magnetic compass.
2. The earth behaves as a magnet. This is the reason for a freely suspended magnet aligning itself along the north-south direction.
a.Who did discover that the earth behaves as a magnet. b.Where do locate the poles of the earth magnets?
c. How will you detect the poles of a bar magnet using its directional property? Ans. a. William Gilbert.
b.The north pole of the earth's magnet is near the geographic south pole and the south pole of the earth's magnet is near the geographic north pole.
c. When a bar magnet is suspended freely, the end that points towards the north direction is its north pole and that points towards the south are the south pole.
3. An hacksaw blade is magnetised by rubbing it with a magnet
several times from A to B as shown in the figure. Identify the poles of
the hacksaw blade (induced magnet).
Ans. A – South pole and B- north pole.
4. What is meant by magnetic lines of force? Which is its direction?
6. See the figure.
Ans. Magnetic lines of force are the imaginary lines indicating
the influence of magnetic force and direction. They are directing from the north pole to the south pole outside the magnet and from the south pole to the north pole inside the magnet.
5. What is magnetic flux density? Where the flux density is maximum in respect of a magnet?
Ans. Magnetic flux density is the number of magnetic lines of force passing normal to unit area. Flux density is maximum at the poles.
a. Which is the device indicated as 'X'.
b.Identify the poles of the magnet marked as A & C.
c.An iron nail is brought at A and then at B. Where the nail is attracted weakly? Why?
Ans. a. Magnetic compass.
8. Magnetic Compass
Compass is a magnetic needle arranged in such a way that it can rotate freely inside an aluminium or plastic case. When placed on a horizontal surface the magnetic needle in it comes to rest quickly in the north-south direction. Hence this instrument is used to find the direction.
9. Find out the errors in marking the magnetic lines of force and draw it properly.
Ans.
i. The directions of the lines force shown in the figure are wrong.
ii. The magnetic lines of force never intersect each other. But in the figure, they intersected.
10. What is a magnetic field?
Ans. The region around a magnet where the magnetic influence is felt is the magnetic field.
11. Substances that are attracted by magnets are magnetic substances. Give examples for magnetic substances.
Ans. Cobalt, Nickel, Iron.
12. What is magnetic induction?
Ans. The phenomenon of a magnetic substance acquiring magnetism due to the presence of a magnet is called magnetic induction.
13. See the figure. If an iron nail is brought near a magnet, it will be magnetised.
a. What is called this phenomenon?
b. Identify the polarities of the ends A&B of the induced magnet(nail).Ans.
a. Magnetic induction.
b. North pole will be induced at A and the south pole at B.
14. Soft iron and steel can be magnetised by induction. What are the differences between their magnetic properties?
Ans. Soft ion can be strongly magnetised very easily. (High susceptibility). But its ability to retain its acquired magnetism is low (Low retentivity).
But steel is not susceptible as soft iron. But its retentivity is higher than that of soft iron.
15. Explain susceptibility and retentivity. Compare soft iron and steel on this basis.
Ans. See answer to Question.13.
16. "Soft iron is better than steel for making a temporary magnet.” Justify this statement.
Ans. This statement is correct. Because soft iron is more susceptible than of steel.
17. What is meant my permeability?
Ans. It is the ability of a substance to pass magnetic lines of force through it.
18. Magnetic compass is not made by arranging a magnetic needle inside a soft iron case. Why?
Ans. The magnetic compass is used to detect the presence and direction of magnetic lines of force. If the magnetic needle is arranged inside a soft iron, the presence of a magnetic field cannot be felt by the needle as the permeability of the soft iron is greater.
19. Answer the following Questions
a. What is an electromagnet?
b. List out the materials for making an electromagnet.
c. Suggest a few methods to increase the strength of an electromagnet.
Ans.
a. It is a temporary magnet. It acts as a magnet only when current is passed through it.
b. Insulated copper wire, Soft iron bar (An iron nail), Battery, Connecting wire.
c. i. Increase the number of turns. ii. Increase the intensity of the current.
iii. Use a material having high susceptibility as a core.
iv. Increase the cross-sectional area of the core as possible.
Ans. The Retentivity of steel is greater than that of soft iron. So it can effectively
retain its acquired magnetism.
21. Two iron nails are placed at the end of a bar magnet as shown in the figures.
 |
| (A) (B) (C) |
a. Find out the correct one.
b. Justify your answer.
Ans.
a. Figure C is correct.
b. As the polarity of the free ends of the nails are similar, it will repel each other.
Ans. Figure. B
23.See the figure.
a. Identify the arrangement.
b. Which of the following is the most suitable one for winding the copper wire?
(Narrow steel bar, thick steel bar, narrow soft iron bar, thick soft iron bar)
Ans. a. Electromagnet.
b. thick soft iron bar
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