Review: Magnets and Currents

The diagram shows a magnet with some plotting compasses. The compasses show the direction of the magnetic field of the magnet. Which plotting compass has the needle pointing in the wrong direction?

• A

• B

• C

• D

The circuit shows one method of magnetising a steel bar.

How can the circuit be altered so that it can be used to demagnetise the magnetised steel bar?

• remove the steel bar from the coil whilst the current is switched on

• replace the d.c. supply with an a.c. supply and gradually reduce the supply voltage to zero

• reverse the polarity of the d.c. supply

• reverse the polarity of the d.c. supply and gradually reduce the supply voltage to zero

A student has a bar magnet. He brings the magnet close to an object. The magnet and the object repel each other. What must the object be?

• another permanent magnet

• any magnetic material

• a block of wood

• a piece of copper

A bar magnet is slowly moved towards an unmagnetised metal object X.

When it is a few centimetres away, the object begins to slide towards the magnet. Why does this happen?

• X is a non-magnetic material and magnetism is induced in it.

• X is a non-magnetic material and magnetism is not induced in it.

• X is a magnetic material and magnetism is induced in it.

• X is a magnetic material and magnetism is not induced in it.

Why is soft iron used for the core of an electromagnet?

• Soft iron easily becomes a permanent magnet.

• Soft iron is a good electrical conductor.

• Soft iron is a poor thermal conductor.

• Soft iron loses its magnetism when the current in the coil is switched off.

An iron ball on a horizontal wooden table rolls near the north pole of a bar magnet which is lying on the table. Which diagram shows the most likely path of the ball, as seen from above the table?

• A

• B

• C

• D

The ends of three metal rods are tested by holding end Q of rod 1 close to the others in turn.

The results are as follows. End Q attracts end R. End Q attracts end S. End Q attracts end T. End Q repels end U. Which of the metal rods is a magnet?

• rod 1 only

• rod 1 and rod 2

• rod 1 and rod 3

• rod 3 only

An iron bar PQ is placed close to a bar magnet. In which diagram does end P become an induced north pole?

• A

• B

• C

• D

The diagram shows a bar magnet at rest on a smooth horizontal surface. A length of soft-iron wire is held parallel to the magnet.

The wire is released. What happens?

• The wire moves away from the magnet.

• The wire moves towards the magnet.

• The wire’s centre stays in its present position and the wire rotates through 90° in a clockwise direction.

• The wire’s centre stays in its present position and the wire rotates through 90° in an anticlockwise direction.

Two circuits are set up as shown. The iron rods are placed close together and are able to move.

What happens to the size of the gap at X when switch S is closed?

• It decreases.

• It decreases then increases.

• It increases.

• It does not change.