Electromagnets and motors — GCSE Physics practice questions

Question 1

A student makes an electromagnet by winding 50 turns of insulated wire around an empty cardboard tube and connecting it to a battery.

Which change would **not** make the electromagnet stronger?

Accepted Answer

D: pushing an aluminium rod into the tube. More turns, a larger current and a soft-iron core all make the magnetic field stronger. Aluminium is not a magnetic material, so an aluminium rod in the tube does not become magnetised and does not strengthen the field.

Question 2

The horn of a car needs a much larger current than the horn button can safely carry. The diagram shows the circuit containing the horn button and the coil of a relay. The relay's contacts are in a separate circuit with the car battery and the horn.

What happens when the driver presses the horn button?

Accepted Answer

A: A small current in the relay coil magnetises its core; the core attracts an iron armature and the contacts close, so a large current flows in the horn.. Pressing the button lets a small current flow in the relay coil. The coil and its core become an electromagnet, which attracts the soft-iron armature. The armature closes the contacts in the second circuit, so a large current flows from the battery through the horn. The button only ever carries the small current.

Question 3

In a fine-beam tube, a beam of electrons moves in a circle in a uniform magnetic field. The diagram shows the circular path; the electrons go round it anticlockwise.

What is the direction of the magnetic field?

Accepted Answer

B: out of the page. At the top of the circle the electrons move to the left, so the conventional current is to the right, and the force must point down towards the centre. The left-hand rule with the current to the right and the force down gives a field out of the page. The field must be at right angles to the plane of the circle, so it cannot point to the centre or along the path.