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VIII SCIENCE -MAGNETISM -ANSWERS FOR TEXTBOOK QUESTIONS
MAGNETISM
EVALUATION
TEXTBOOK EXERCISES
I. Choose the best answer.
1. A magnet attracts __________.
a) wooden materials
b) any metal
c) copper
d) iron and steel
Ans: d) iron and steel
2. One of the following is an example for a permanent magnet.
a) Electromagnet
b) Mumetal
c) Soft iron
d) Neodymium
Ans: d) Neodymium
3. The south pole of a bar magnet and the north pole of a U-shaped magnet will
__________.
a) attract each other
b) repel each other
c) neither attract nor repel each other
d) None of the above
Ans: a) attract each other
4. The shape of the Earth’s magnetic field resembles that of an imaginary __________.
a) U-shaped magnet
b) straight conductor carrying current
c) solenoid coil
d) bar magnet
Ans: d) bar magnet
5. MRI stands for __________.
a) Magnetic Resonance Imaging
b) Magnetic Running Image
c) Magnetic Radio Imaging
d) Magnetic Radar Imaging
Ans: a) Magnetic Resonance Imaging
6. A compass is used for __________
a) plotting magnetic lines
b) detection of magnetic field
c) navigation
d) All of these
Ans: d) All of these
II. Fill in the blanks.
1. The magnetic strength is __________ at the poles.
Ans: maximum
2. A magnet has __________ magnetic poles.
Ans: two
3. Magnets are used in __________ for generating electricity.
Ans: dynamos
4. __________ are used to lift heavy iron pieces.
Ans: Electromagnets
5. A freely suspended bar magnet is always pointing along the __________ north-south direction.
Ans: geographic
III. Match the following.
Ans:
IV. Consider the statements given below and choose the correct option.
1. Assertion: Iron filings are concentrated more at the magnetic poles.
Reason: The magnets are so sharp.
2. Assertion: The Earth’s magnetic field is due to iron present in its core.
Reason: At a high temperature a magnet loses its magnetic property or magnetism.
a. Both assertion and reason are true and reason is the correct explanation of
assertion.
b. Both assertion and reason are true, but reason is not the correct explanation of assertion.
c. Assertion is true, but reason is false.
d. Assertion is false, but reason is true.
Ans:
1. b. Both assertion and reason are true, but reason is not the correct explanation of assertion.
2. d. Assertion is false, but reason is true.
V. Answer briefly.
1. Define magnetic field.
Magnetic field is defined as the space around a magnet in which its magnetic effect or influence is observed.
It is measured by the unit tesla or gauss (1 tesla =10,000 gauss).
2. What is artificial magnet? Give examples.
Magnets that are made by people in the laboratory or factory are called artificial magnets. These are also known as man made magnets, which are stronger than the natural magnets.
Example:Bar magnets, U-shaped magnets, horseshoe magnets, cylindrical magnets, disc magnets, ring magnets and electromagnets.
3. Distinguish between natural and artificial magnets?
4. Earth acts as a huge bar magnet. Why? Give reasons.
❂ A freely suspended magnetic needle at a point on the Earth comes to rest approximately along the geographical north - south direction.
❂ This shows that the Earth behaves like a huge magnetic dipole with its magnetic poles located near its geographical poles.
❂ The north pole of a magnetic needle approximately points towards the geographic north (NG).
❂ The magnetic north pole of the needle is attracted by the magnetic south pole of the Earth (Sm), which is located at the geographic north (NG).
❂ The magnetic south pole of the needle is attracted by the magnetic north pole of the Earth (Nm), which is located at the geographic south (SG).
❂ The magnitude of the magnetic field strength at the Earth’s surface ranges from 25 to 65 microtesla.
5. How can you identify non-magnetic materials? Give an example of a nonmagnetic material.
Materials which are not attracted by magnets are called non-magnetic materials.
Example: Glass, Wood, Rubber, Plastic etc.
VI. Answer in detail.
1. List out the uses of magnets.
The uses of magnets are
💮 In ancient times the magnet in the form of ‘direction stone’ was used by seamen to find the directions during a voyage.
💮 Nowadays, magnets are used to generate electricity in dynamos.
💮 Electromagnets are used in our day to day life.
💮 They are used in electric bells and electric motors.
💮 They are used in loudspeakers and microphones.
💮 An extremely powerful electromagnet is used in the fast moving Maglev train to remain floating above the tracks.
💮 In industries, magnetic conveyor belts are used to sort out magnetic substances from scraps mixed with non-magnetic substances.
💮 In banks, the magnets enable the computers to read the MICR numbers printed on a cheque.
💮 The tip of the screw drivers are made slightly magnetic so that the screws remain attached to the tip.
💮 At hospitals, MRI (Magnetic Resonance Imaging) is used to scan the specified internal organ. An extreme from electromagnet is used in it.
2. How will you convert a ‘nail’ into a temporary magnet?
Temporary magnets are produced with the help of an external magnetic field. They lose their magnetic property as soon as the external magnetic field is removed.
💮 Take an iron nail and Slide it along its length in one direction slowly till the other end is reached.Repeat the process 20 to 30 times.
💮 The magnet has to be moved in one direction only.
💮 Avoid the swiping of the magnet back and forth. Now, bring the iron nail near the steel pins.
💮 The steel pins stick to the iron nail because nail has become a temporary magnet.
3. Write a note on Earth’s magnetism
💮 Earth has been assumed or imagined by the scientists as a huge magnetic dipole.
💮 The south pole of the imaginary magnet inside the Earth is located near the geographic north pole and the north pole of the earth’s magnet is located near the geographic south pole.
💮 The line joining these magnetic poles is called the magnetic axis.
💮 The magnetic axis intersects the geographic north pole at a point called the north geomagnetic pole or northern magnetic pole.
💮 It intersects the geographic south pole at a point called the south geomagnetic pole or southern magnetic pole.
💮 The magnetic axis and the geographical axis (axis of rotation) do not coincide with each other.
💮 The magnetic axis of the Earth is inclined at an angle of about 10° to 15° with the geographical axis.
💮 The exact cause of the Earth’s magnetism is not known even today. However, some important factors, which may be the cause of the Earth’s magnetism, are. • Masses of magnetic substances in the Earth
• Radiations from the Sun
• Action of the Moon
VII. Higher Order Thinking Questions.
1. Though Earth is acting as a huge bar magnet it is not attracting other ferromagnetic materials. Why? Give reasons.
Ans:
💮 The Earth itself is not a magnet.
💮 The Earth has a magnetic field which is produced deep in the interior by electrical currents in the molten part of the core.
💮 Earth's magnetic field, at the Earth's surface, ranges from 25 to 65 microtesla (0.25 to 0.65 gauss).
💮 The field is so weak that it does not normally attract any ferromagnetic materials to any degree detectable by direct observation.
💮 It is strong enough to rotate a small magnet, like a compass needle, balanced with little friction on a pointed support.
2. Why it is not advisable to slide a magnet on an iron bar back and forth during
magnetising it?
💮 The magnet has to be moved in one direction only.
💮 If it is moved/swiped in opposite directions it will cancel the magnetic power of both magnet and iron bar.
3. Thamizh Dharaga and Sangamithirai were playing with a bar magnet. They put the magnet down and it broke into four pieces.How many poles will be there?
💮 Each broken piece functions like a separate magnet.
💮 Therefore, altogether they will have eight poles.
