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physics Physics For High Schools (KS4)
 
Solved problems for the chapter:
 

Methods of Magnetization and Demagnetization

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Example 1.

How can a permanent magnet be demagnetized?


  • Cool the magnet for a long time

  • Hit the magnet repeatedly with a hammer

  • Leave the magnet in a coil which carries direct current

  • Pass a small current through the magnet

    
Example 2.

How many of the following methods could be used to demagnetise a piece of steel?


  • Heating it until it is red hot

  • Pulling it from a coil that is carrying an alternating current

  • Placing it in an east-west direction and hammering it

  • Putting it in a coil carrying a direct current


  • 1

  • 2

  • 3

  • 4

    
3.

A student demagnetises a magnetized steel bar. He places the bar in a solenoid connected to a power supply. He then removes the bar from the solenoid. Which row indicates the most effective way of demagnetizing the bar?


 
type of power supply
speed to remove bar
A
a.c
fast
B
a.c
slow
C
d.c
fast
D
d.c
slow
    
4.

A solenoid carrying a current is used to demagnetise a bar magnet.




Which conditions achieve demagnetization?


 
Current through solenoid
Movement of the bar magnet
 
A
a.c
around the solenoid quickly
 
B
a.c
through the solenoid slowly
 
C
d.c
around the solenoid quickly
 
D
d.c
through the solenoid slowly
 
    
5.

What is the most effective method of demagnetising a bar magnet?


  • Placing the magnet in a solenoid carrying a large a.c and gradually decreasing the current

  • Placing the magnet in a solenoid carrying a large d.c and gradually decreasing the current

  • Placing the magnet in a solenoid that produces a magnetic field in the opposite direction to the magnet

  • Placing the magnet next to an identical bar magnet with its poles in the opposite direction

    
6.

A student wishes to demagnetise a steel bar. He uses the apparatus shown.




Which type of supply should the student use and what should he do with the steel bar?


 
supply
what to do with the steel bar
 
A
d.c
keep it inside the coil
 
B
d.c
slowly remove it from the coil
 
C
a.c
keep it inside the coil
 
D
a.c
slowly remove it from the coil
 
    
7.

A bar magnet can be demagnetized by hammering it for a long time or by slowly removing it from a coil connected to a power supply.


Which row completes the descriptions of how these processes are carried out?


 
hammering bar magnet for a long time with
slowly removing bar magnet from a coil connected to
A
magnet aligned E-W
a d.c power supply
B
magnet aligned E-W
an a.c power supply
C
magnet aligned N-S
a d.c power supply
D
magnet aligned N-S
an a.c power supply
    
8.

A coil is connected to a battery and a soft iron bar is near to it.




The current is then reversed by reversing the battery connections.


How does the soft iron bar behave in the two cases?


 
with the battery as shown
with the battery reversed
 
A
attracted to the coil
attracted to the coil
 
B
attracted to the coil
repelled from the coil
 
C
repelled from the coil
attracted to the coil
 
D
repelled from the coil
repelled from the coil
 
    

9.

Which procedure may be used to demagnetise a steel bar?


  • Cooling it in a freezer

  • Earthing it with a copper wire

  • Placing it in a solenoid carrying a large direct current (d.c)

  • Striking it repeatedly with a hammer

    
10.

Which statement about magnetism is correct?


  • Aluminum is a ferrous metal

  • A steel magnet can be demagnetized by heating it

  • The core of an electromagnet is usually made of steel

  • The magnetic field lines around a bar magnet are evenly spaced

    
11.

An old expensive steel watch becomes magnetized. The owner wants to use the watch again, so he must demagnetise the watch. What is the best method to do this?


  • Heat it until it glows red hot

  • Pass direct current through it

  • Place it in a plastic bag and put the bag in hot water for several hours

  • Place it in a solenoid that carries alternating current and then slowly remove it

    
12.

A student wishes to use a magnetising coil to make a magnet from a piece of metal.




Which metal should be used?


  • Aluminum

  • Copper

  • Brass

  • Steel

    
13.

What will be the least successful method of magnetising a steel bar?


  • Insert the bar in a coil with a large direct current

  • Place the bar at right angles to a weak magnetic field and hit it with a hammer

  • Place the bar parallel to a strong magnetic field, heat it and let it cool

  • Stroke the bar with a magnet

    
14.

A piece of steel is slightly magnetised. It is hit several times with a hammer. What effect will this have on the steel?


 
the steel is parallel to a strong magnetic field
the steel is right angles to a weak magnetic field
 
A
it becomes magnetised more strongly
it becomes magnetised more strongly
 
B
it becomes magnetised more strongly
it loses its magnetisation
 
C
it loses its magnetisation
it becomes magnetised more strongly
 
D
it loses its magnetisation
it loses its magnetisation
 
    
15.

A permanent magnet is to be made. Which row gives the correct material to use and the correct reason for choosing this material?


 
material
reason
 
A
soft iron
easy to demagnetise
 
B
soft iron
hard to demagnetise
 
C
steel
easy to demagnetise
 
D
steel
hard to demagnetise
 
    
16.

Which action will not magnetise a rod?


  • Placing a copper rod inside a coil carrying a direct current

  • Stroking a steel rod with a permanent magnet

  • Hammering a steel rod aligned with the earth’s magnetic field

  • Placing a soft iron rod close to a permanent magnet

    
17.

A steel magnet is placed inside a coil of wire. Which method is used to demagnetise the magnet?


  • Connect the coil to an a.c power supply

  • Connect the coil to a d.c power supply

  • Connect the coil to an a.c power supply and slowly remove the magnet from the coil.

  • Connect the coil to a d.c power supply and slowly remove the magnet from the coil

    
18.

Which method is used to demagnetise a bar magnet?


  • Heat it with a Bunsen burner

  • Lower it into water

  • Place it in a metal box

  • Suspend it in a sling

    

19.

The diagram shows apparatus that can be used to make a magnet.




Which metal and which power supply are used to make a permanent magnet?


 
Metal
Power supply
 
A
Aluminium
6 V a.c
 
B
Aluminium
6 V d.c
 
C
Steel
6 V a.c
 
D
Steel
6 V d.c
 
    
20.

A student wishes to magnetize two steel bars X and Y. She places bar X in a coil connected to a d.c power supply. She places bar Y parallel to a magnetic field and hammers it.


Which bar becomes magnetised?


  • X and Y

  • X only

  • Y only

  • Neither X nor Y