AQA GCSE Combined Science (8464), Higher tier · Physics › Magnetism and electromagnetism › The motor effect
Practise Electric motors (HT only). 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.
A coil of wire carrying a current in a magnetic field tends to rotate: this is the basis of an electric motor. You need to explain, using the motor effect and Fleming's left-hand rule, why the coil turns, which way it turns and how it keeps turning. Higher tier only. Expect 'explain' questions from 2 marks up to 6-mark extended answers, usually with a diagram of a simple d.c. motor.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
4
State that a coil carrying current rotatesA coil of wire carrying a current in a magnetic field tends to rotate.
5
Explain why the coil's sides feel opposite forcesThe current is in opposite directions in the two sides, so the forces on them are in opposite directions.
6
Describe how to reverse or speed up motorsReverse the current or the field to reverse it; increase the current, field strength or number of turns to speed it up.
6
Find which way a motor coil turnsUse Fleming's left-hand rule on each side of the coil to find the forces, then decide clockwise or anticlockwise.
7
Explain the role of the split-ring commutatorIt reverses the current in the coil every half turn, so the coil keeps turning in the same direction.
8
Explain why the coil keeps turning past verticalWhen the coil is vertical the forces give no turning effect, but its momentum carries it past, just as the commutator reverses the current.
Notes
Why the coil turns
A simple d.c. motor has a rectangular coil of wire on an axle, between the poles of a magnet.
The current goes one way along one side of the coil and the opposite way along the other side.
By the motor effect, each side has a force on it. The forces are in opposite directions (one up, one down), on opposite sides of the axle, so the coil rotates.
Use Fleming's left-hand rule on each side to find the direction of rotation.
A side of the coil that is parallel to the field lines (e.g. the short end side when the coil is horizontal) has no force on it.
Keeping it turning: the split-ring commutator grade 7+
Without a commutator, the coil would only turn until it was vertical. Past that point the forces would push it back, so it would rock and then stop instead of spinning.
The split-ring commutator is a ring split into two halves, one joined to each end of the coil. Two brushes press against it to carry the current in and out.
Every half turn, the commutator swaps which end of the coil is connected to each brush. This reverses the current in the coil.
So the side of the coil nearer the north pole always has current in the same direction, and the coil keeps turning the same way.
When the coil is vertical, the two forces act along a line through the axle, so they give no turning effect. The coil's momentum carries it past this point, which is when the current reverses. grade 8+
Changing the motor
To reverse the direction of rotation: reverse the current (swap the connections to the supply) or reverse the magnetic field (swap the poles).
To make it turn faster (bigger forces): increase the current, use a stronger magnet, or use more turns of wire on the coil.
Cheatsheet
Coil carrying a current in a magnetic field → rotates (the basis of an electric motor)
Current in opposite directions in the two sides → forces in opposite directions → rotation
Direction of rotation: Fleming's left-hand rule on each side
Split-ring commutator: reverses the current in the coil every half turn
Brushes: make contact with the spinning commutator
Reverse the rotation: reverse the current or reverse the field
Faster: bigger current, stronger magnet, more turns
How to answer each type of question
Explain why the coil of a motor rotates
2 to 3 marks6
Say each side of the coil is a current-carrying wire in a magnetic field, so it has a force on it (motor effect).
Say the current is in opposite directions in the two sides.
So the forces are in opposite directions, and the coil turns.
Example. A simple electric motor has a rectangular coil between the poles of a magnet. The coil is horizontal and there is a current in it. Explain why the coil starts to rotate. [3 marks]
Show the model answer
Each side of the coil is a current-carrying conductor in a magnetic field, so it has a force on it (the motor effect) (1). The current is in opposite directions in the two sides (1), so the forces are in opposite directions (one up, one down), which makes the coil turn (1).
Use Fleming's left-hand rule to find the direction of rotation
2 to 3 marks6
Mark the field direction from N to S.
Use the left-hand rule on one side of the coil to find its force, then on the other side (it will be opposite).
Decide whether this turns the coil clockwise or anticlockwise.
Example. A motor coil is viewed from one end of its axle. The coil is horizontal. The north pole of the magnet is on the left and the south pole is on the right. The current is into the page in the left-hand side of the coil, and out of the page in the right-hand side. Explain which way the coil starts to turn. [3 marks]
Show the model answer
The field is from left to right (N to S) (1). By Fleming's left-hand rule, the force on the left-hand side is downwards and the force on the right-hand side is upwards (1). So the coil turns anticlockwise (1).
State how to change the speed or direction of a motor
1 to 3 marks6
Faster: bigger current, stronger magnet, more turns on the coil.
Opposite direction: reverse the current, or reverse the magnetic field. Not both.
Example. (a) Give two changes that would make a simple d.c. motor spin faster. [2 marks] (b) State one change that would make the motor spin in the opposite direction. [1 mark]
Show the model answer
(a) Any two, 1 mark each: increase the current (or the potential difference); use a stronger magnet; use a coil with more turns. (b) Reverse the current (swap the connections) or reverse the magnetic field (swap the poles) (1).
6-mark: explain how a d.c. motor keeps turning
6 marks8
Start with the motor effect: each side of the coil has a force on it.
Explain why the forces are opposite and make the coil turn.
Explain what the split-ring commutator does and when (every half turn).
Link it back: so the coil keeps turning in the same direction.
Write in full sentences and in a logical order: the whole answer is marked for its quality, not just point by point.
Example. A simple d.c. motor has a coil between the poles of a magnet. The coil is connected to a d.c. supply through a split-ring commutator and brushes. Explain how the motor produces continuous rotation in one direction. [6 marks]
Show the model answer
A top-level answer includes most of these points in a logical order: there is a current in the coil, which is in the magnetic field of the magnet; each side of the coil at right angles to the field has a force on it (the motor effect); the current is in opposite directions in the two sides, so the forces are in opposite directions (Fleming's left-hand rule); the forces act on opposite sides of the axle, so the coil turns; when the coil is vertical there is no turning effect, but its momentum carries it on; the split-ring commutator reverses the current in the coil every half turn; so the forces keep turning the coil the same way.
Shortcuts and memory tricks
A motor is two motor-effect wires side by side: opposite currents, opposite forces, rotation.
The commutator swaps the connections every half turn: think 'commute = switch'.
Faster motor: more Current, more Magnet strength, more turns.
Where marks are lost
Saying the commutator reverses the direction of rotation: it keeps the rotation in the same direction by reversing the current.
Saying the commutator reverses the current every full turn: it is every half turn.
Saying the magnet 'attracts' the coil: the force is the motor effect on the current in the coil.
Forgetting that the currents in the two sides of the coil are in opposite directions.
Writing 'a bigger magnet' instead of 'a stronger magnet'.
Exam technique
Use the key phrases: 'motor effect', 'current in opposite directions', 'forces in opposite directions', 'rotates' and 'every half turn'.
If a diagram shows the current, state the direction of rotation (clockwise or anticlockwise) and say you used Fleming's left-hand rule.
Refer to 'the coil', 'the current' and 'the magnetic field' rather than 'it'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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.