8th Science Guide Unit 7 Magnetism

8th Std Science Guide Lesson 7 Magnetism | English Medium

8th Science Guide Unit 7 Magnetism book back answers. 8th Standard Science Guide English Medium Book Back Answers. 8th Science Samacheer kalvi guide Tamil Medium and English Medium Book Back and additional questions and answers. 8th Text Book download pdf. 8th std All Subject Guide. 

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8th Science Guide Unit 7 Magnetism

I. Choose the best answer.

1. A magnet attracts __________.

1. wooden materials
2. any metal
3. copper
4. iron and steel

Ans : iron and steel

2. One of the following is an example for a permanent magnet.

1. Electromagnet
2. Mumetal
3. Soft iron
4. Neodymium

Ans : Neodymium

3. The south pole of a bar magnet and the north pole of a U-shaped magnet will __________.

1. attract each other
2. repel each other
3. neither attract nor repel each other
4. None of the above

Ans : attract each other

4. The shape of the Earth’s magnetic field resembles that of an imaginary __________.

1. U-shaped magnet
2. straight conductor carrying current
3. solenoid coil
4. bar magnet

Ans : bar magnet

5. MRI stands for __________.

1. Magnetic Resonance Imaging
2. Magnetic Running Image
3. Magnetic Radio Imaging
4. Magnetic Radar Imaging

Ans : Magnetic Resonance Imaging

6. A compass is used for __________

1. plotting magnetic lines
2. detection of magnetic field
3. navigation
4. All of these

Ans : 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 : Magnet
5. A freely suspended bar magnet is always pointing along the __________ north-south direction Ans : geographic

III. Match the following.

1. Magnetite – Magnetic lines
2. A tiny pivoted magnet – Natural magnet
3. Cobalt – Compass box
4. Closed curves – Ferromagnetic material
5. Bismuth – Diamagnetic material

Ans: 1 – b, 2 – c, 3 – d, 4 – a, 5 – e

IV. Choose the correct option.

1. Both assertion and reason are true and reason is the correct explanation of assertion.
2. Both assertion and reason are true but reason is not the correct explanation of assertion.
3. Assertion is true but reason is false.
4. Assertion is false but reason is true.

1. Assertion : Iron f lings are concentrated more at the magnetic poles

Reason : The magnets are so sharp

Ans : Both assertion and reason are true but reason is not the correct explanation of assertion.

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.

Ans : Assertion is false but reason is true.

V. Answer briefly.

1. What is 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).

example

• Bar magnets
• U-shaped magnets
• Horseshoe magnets
• Cylindrical magnets
• Disc magnets
• Ring magnets
• Electro magnets

2. What is artificial magnet? Give examples.

• Magnets that are made by people in the laboratory or factory are called artificial magnets.

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. Te north pole of a magnetic needle approximately points towards the geographic north (NG).
• Thus, it is appropriate to say that 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.
• Also, 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 micro tesla.

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.

Ex. : Wood, Glass, Rubber, Plastic

VI. Answer in detail.

1. List out the uses of magnets in day to day life?

• In ancient times the magnet in the form of ‘direction stone’ helped seamen to find the directions during a voyage.
• Nowadays, magnets are used to generate electricity in dynamos.
• Magnets, especially 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.
• Magnets are used in computer in its storing devices such as hard disks.

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. They are made from soft iron.
• Soft iron behaves as a magnet under the influence of an external magnetic field produced in a coil of wire carrying a current.
• But, it loses the magnetic properties as soon as the current is stopped in the circuit.
• Magnets used in electric bells and cranes are the examples of temporary magnets.

3. Write a note on Earth’s magnetism.

• Magnetism Book Back Solution 2021 – 2022
• Earth has been assumed or imagined by the scientists as a huge magnetic dipole.
• However, the position of the Earth’s magnetic poles is not well defned in the Earth.
• 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 ax is 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 as follows.
• Masses of magnetic substances in the Earth
• Radiations from the Sun
• Action of the Moon.