AQA GCSE Physics (8463), Higher tier · Magnetism and electromagnetism › The motor effect
Practise Electric motors. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Electric motors use the motor effect. Give two ways of making an electric motor turn faster.
Any two: increase the current; use a stronger magnet; use more turns on the coil.
Name the part of a simple d.c. motor that reverses the current in the coil every half turn.
The split-ring commutator.
State the purpose of the brushes in a simple d.c. motor.
They keep electrical contact between the circuit and the rotating commutator.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Electric motors use the motor effect.
(a) A coil of wire carrying a current is placed in a magnetic field. What does the coil tend to do? Tick (✓) one box.[1]
Rotate
Stay still
Become a permanent magnet
Move towards the nearest pole
(b) Give two ways of making an electric motor turn faster.[2]
(c) Give one way of reversing the direction in which the motor turns.[1]
Show the answer and mark scheme
(a)Answer: Rotate
(b)Answer: Any two: increase the current; use a stronger magnet; use more turns on the coil.
increase the current / potential difference
use a stronger magnet
use a coil with more turns
(c)Answer: Reverse the current (or the magnetic field).
reverse the current / reverse the magnetic field (swap the poles of the magnet)
Question 2Medium2 marks
A motor's coil is wound around a soft iron core (this combination is sometimes called the armature).
Explain why adding an iron core inside the coil increases the turning effect (moment) of the motor, for the same current.[2]
Show the answer and mark scheme
Answer: The magnetised iron core makes the coil a stronger electromagnet (a stronger field at the coil), so the forces on the coil, and therefore the moment, are bigger.
the iron core becomes magnetised, making the coil a much stronger electromagnet / concentrating the magnetic field where the sides of the coil are
so the forces on the coil are bigger (greater attraction and repulsion between the coil's poles and the magnet's poles / a greater flux density in F = BIl), giving a bigger moment
Question 3Hard6 marks
A simple dc motor has a rectangular coil of wire between the poles of a permanent magnet. The coil is connected to a dc supply through a split-ring commutator and two brushes.
Explain how the motor produces continuous rotation.[6]
Show the answer and mark scheme
Answer: Level-of-response answer: forces on the two sides of the coil (motor effect) are opposite and produce a moment; the split-ring commutator reverses the current every half turn so the rotation continues in one direction.
there is a current in the coil (through the brushes and split-ring commutator)
the coil is in the magnetic field between the poles of the magnet
the sides of the coil at right angles to the field experience a force (motor effect)
the current in the two sides is in opposite directions, so the forces are in opposite directions (Fleming's left-hand rule)
the forces act on opposite sides of the axle, producing a moment, so the coil turns
when the coil is vertical there is no moment, but its momentum carries it past this position
the split-ring commutator reverses the direction of the current in the coil every half turn
so the forces keep turning the coil in the same direction
the speed can be increased by increasing the current, the strength of the magnetic field or the number of turns on the coil
Marked with levels of response: the full level descriptors are in the app.