Magnetism

Question 1

Properties of magnet

Answer 1

1. Magnets attract objects made out of ferromagnetic materials.
2. Magnetic poles always occur in pairs of a north pole and a south pole.
3. Unlike poles of a magnet attract each other while like poles of a magnet repel each other.
4. If a bar magnet is suspended by a thread and free to rotate, the north pole will move towards the North Pole of the earth and vice versa.

Question 2

Ferromagnetic materials

Answer 2

1. Can become induced magnets when placed in a magnetic field
2. Can also be magnetised into permanent magnets.

Question 3

Soft magnetic materials

Answer 3

Easy to magnetise and demagnetise (iron)

Question 4

Hard magnetic materials

Answer 4

Hard to magnetise and demagnetise (steel)

Question 5

Magnetic induction

Answer 5

Process where an object made of magnetic material becomes a magnet when it is near or in contact with a magnet

Question 6

Magnetisation by stroking

Answer 6

• for hard magnetic materials only
• magnet needs to be lifted sufficiently high to prevent the magnetism induced from being cancelled when the magnet moves along the steel bar
• divided touch produces a stronger magnet

Question 7

Magnetisation by electrical method

Answer 7

When current flows through the solenoid, a strong magnetic field is produced. The magnetic field passes trough the steel bar, causing the magnetic domains to be aligned in the same direction as the magnetic field set up by the solenoid.
- more effective than stroking

Question 8

Demagnetisation by heating and hammering

Answer 8

The magnet is hammered vigorously while lying on the east-west direction.
Magnetism can also be destroyed if the magnet is heated to red-hot and left to cool on the east-west direction

Question 9

Demagnetisation using ac circuit

Answer 9

Connect a solenoid to an ac circuit.
Place the magnet in the solenoid.
When the ac is flowing, slowly withdraw the magnet in the east-west direction until its far away from the solenoid.
The magnet will be magnetised one way and then the opposite way repeatedly, its magnetism becomes weaker and weaker until the magnet is completely demagnetised.

Question 10

Magnetic field

Answer 10

Region where a magnetic material experiences a magnetic force

Question 11

Electromagnets

Answer 11

• An electromagnet consists of a coil wounded on the core of soft magnetic material e.g. soft iron.
• The core is magnetised when the circuit is closed and current flows through the coil.
• The core loses magnetism when the circuit is open.
• The strength of the electromagnet is increased by:
1. using cells of larger e.m.f. (hence passing a larger current through the coil)
2. increasing the number of turns per unit length of the coil
3. inserting a soft iron core made of soft magnetic material (if the core is not present yet)