PHYSICS NOTES - CH - 6 - MAGNETISM - UNIT 2 - Properties and Uses of Magnets (Ana NOTES)

Formal Notes: Magnetism- UNIT - 2 

- Properties and uses of Magnets.

Q1 (a) You are provided with a 15 cm long steel strip. By using a permanent magnet, explain how you will magnetise the steel strip.

Ans:

• Take the thin steel strip about 15 cm long. Mark one side as 'A' and the other one as 'B'.

• Place the strip AB along one edge of the table.

• Rub the magnet along the strip from till point B.

• Lift the magnet and place it again on point A rubbing it along till point B.

• Repeat this 40 times. And the steel strip will get converted into a magnet.

(b) How will you verify the magnetic poles formed on the steel strip? 

Ans:

• Bring the end A of the strip to the north pole of a freely pivoted magnetic needle.

• The north pole of a freely suspended magnetic needle is repelled, which proves that end A of the steel strip changed into North pole of a magnet and hence end B becomes South pole.

(c) Suggest a simple way of demagnetization of above steel strip? 

Ans: If we hammer the steel strip of times of heat it to a very high temperature it gets demagnetised.

Q.2 Why is repulsion considered as the surest test of magnetism? 

Ans: It is because the attraction can be caused between two unlike poles of two magnets or between the magnet and the magnetic substance. However repulsion is caused when two similar poles of two magnets come close to each other. So, repulsion is the surest test of magnetism.

Q.3 You are provided with a 5cm and 1/2 cm thick soft iron rod. How will you convert it into a electromagnet?

Ans: 

• Take a soft iron piece rod of 5cm long and 1/2 cm thick.

• Take an insulated copper wire leaving some of the length of it, wind the wire around the soft iron rod.

• Bare the ends of the copper wire from insulation. Fix each end of the bare copper wire on the terminal of a dry cell.

• Bring the above arrangement near the iron paper clips. It is observed that the paper clips cling on to the wire.

• This proves that the wire changes the iron rod into a magnet when current flows through it.

• Now remove the wire from the dry cell. It is found the paper clips falls down.

• From this we conclude the iron rod behaves like a magnet as long as current flows through the wire.

• A magnet which operates only when electric current flows around the iron called electromagnet.

(Fig )

Q.4 How are bar magnets stored? Explain, especially the role of magnetic keepers.

Ans. Storing of magnets:

• To prevent demagnetization, the bar magnets are arranged in pairs.

• Bar magnets are placed with opposite poles facing each other.

• Two soft iron bars are placed with opposite poles facing each other.

• This system forms a giant chain and protects the magnets.

Q.5 A box containing coins made out of nickel and copper. How will you separate the coins using a powerful bar magnet?

Ans: Roll a powerful bar magnet in the pile of coins. Only the coins containing nickel will stick to the magnet, but not the copper coins. Repeat this process two to three times till all nickel coins are removed.

Q.6 State 6 important properties of a bar magnet. 

Ans: 1. A bar magnet attracts magnetic substances [like iron, steel, cobalt and nickel] towards itself. 

2. When suspended freely it points in north-south direction. 

3. Like poles of a bar magnets repel each other. 

4. Unlike poles of a bar magnets attract each other. 

5. When a bar magnet is rubbed over an unmagnetised piece of iron or steel, it changes into a magnet. 

6. When a bar magnet is broken, then each piece is a complete magnet.

Q.7 State four uses of magnet/electromagnets. 

Ans: Uses for magnets:

• Making magnetic stickers/toys

• Magnets are used in compass needles Uses for electromagnets:

• Electromagnets are used in fans, motors, mixers, room coolers

• Electromagnets are used for lifting heavy loads.

Q.8 Distinguish between Permanent and temporary magnets.

Permanent Magnets	Temporary Magnets
1. It doesn't lose magnetism when the cause producing it is removed.	1. It loses its magnetism as soon as the cause producing it is removed.
2. It is generally made of steel.	2. It is generally made of soft iron.
3. Fixed north and south poles.	3. The position of north and south can be changed.
4. Its magnetic strength is fixed.	4. Magnetic strength can be changed.

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Q.9 Discuss the ways of de-magnetisation?

Ans: A magnet can be demagnetised by:

• By heating the magnet to a high temperature.

• By hammering the magnet frequently.

• By flowing alternating current through a coil around the magnet is kept in east-west direction.

• By rough handling.

• By dropping the magnet frequently.