Electric motors (HT only) — AQA GCSE Physics
Test yourself on Electric motors (HT only) with AQA GCSE practice questions.
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Electric motors (HT only) explained
Current in the two long sides of the coil runs in opposite directions, and both sides sit in the same magnetic field, so the motor effect force F = B I l pushes one side up and the other side down.
Read the full explanation
Two equal forces acting in opposite directions on either side of the axle form a couple, and a couple is a turning effect, so the coil rotates. Fleming's left hand rule fixes which way round: first finger field, second finger current, thumb motion. The force on a side is largest when that wire lies at right angles to the field lines. As the coil turns the forces stay the same size, but the perpendicular distance from the axle shrinks, so the turning effect is greatest when the plane of the coil lies along the field and falls to zero when the coil reaches the position at right angles to it.
This is the basis of an electric motor.
A practical direct current motor turns that idea into hardware: a coil on an axle between the poles of a permanent magnet, fed through a split-ring commutator and two carbon brushes. Without the commutator the coil would swing round to the vertical and stop, because beyond that point the same forces would push it back again. The split ring swaps the two ends of the coil over every half turn, so the current in whichever side is now on the left always runs the same way, the force on that side always points the same way, and the coil keeps turning in one sense. Making the current larger, using a stronger magnet, or winding on more turns all increase the turning effect. Reversing the current, or swapping the magnetic poles over, reverses the rotation, but doing both together changes nothing.
Students should be able to explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.
Examiners want a chain here, not a list. Start with the current in the coil, then say each long side is a conductor lying at right angles to the field, so each one feels a motor effect force. Use Fleming's left hand rule to say those forces are in opposite directions, one side up and the other down. Two opposite forces either side of the axle give a turning effect, so the coil rotates. Then deal with what happens as the coil passes the vertical: the split-ring commutator swaps which brush each end of the coil touches, so the current through each side reverses at the moment that side crosses over. The force on the side now rising still points upwards, so the coil carries on turning the same way instead of rocking back. Every link in that chain is usually worth a mark.
Your focus
- Predict the direction of the force on each side of a current-carrying coil using Fleming's left hand rule.
- Explain why two equal forces acting in opposite directions either side of an axle produce rotation rather than movement in a straight line.
- Describe how the turning effect changes as the coil rotates, and identify the position where it falls to zero.
Show all 11 objectives
- Identify the orientation of a conductor in a magnetic field that gives the largest force.
- Name the parts of a direct current motor and state the job each one does.
- Explain why a motor built with a slip ring instead of a split ring would come to rest after half a turn.
- Predict the effect on a motor of reversing the current, swapping the magnetic poles, or doing both at once.
- Suggest three changes that would make a given motor turn faster and justify each one.
- Construct a step by step explanation of a motor running from current, to force, to turning effect, to commutator action.
- Annotate a diagram of a motor with the directions of the field, the current and the force on each side of the coil.
- Explain why the direction of rotation stays the same even though each side of the coil changes sides twice per turn.
Electric motors (HT only) exam tips
Marking Points
- one mark for stating that the current in the two sides of the coil is in opposite directions
- one mark for a force acting on each side, in opposite directions, because of the motor effect
- one mark for the two forces forming a couple that turns the coil about its axle
- one mark for the force on a conductor being a maximum when it is at right angles to the magnetic field
- one mark for a coil carrying a current in the field of a permanent magnet
- one mark for the split-ring commutator reversing the current in the coil every half turn
- one mark for explaining that this keeps the force on each side acting the same way, so rotation continues
- one mark for a named way of increasing the speed, such as a larger current, more turns or a stronger magnet
- one mark for a force acting on each side of the coil because it carries a current across a magnetic field
- one mark for the two forces acting in opposite directions, one up and one down
- one mark for the pair of forces producing a turning effect about the axle
- one mark for the commutator reversing the current in the coil every half revolution
- one mark for explaining that the reversal keeps the rotation going in the same direction
Examiner Tips
- 💡Draw current arrows on both sides of the coil before using the left hand rule; they point opposite ways, and that is where the turning comes from.
- 💡In an explain answer, name the rule you used and state the field direction you assumed, north to south.
- 💡A turning effect needs two forces, so an answer that mentions only one force rarely gets past the first mark.
- 💡Learn the parts list, coil, permanent magnet, split-ring commutator and brushes, and name them in any explanation.
- 💡If asked how to make the motor turn faster, change one thing at a time and say why it increases the force.
- 💡A question asking why the coil does not stop is always about the commutator; say what it swaps and how often.
- 💡Label a sketch with the field direction, the current direction and the two force arrows, then write the answer from the sketch.
- 💡Use the words force, opposite directions, turning effect and commutator; each of them tends to carry a mark.
- 💡Check the mark allocation: a five mark explanation needs five separate linked statements.
Common Mistakes
- saying the whole coil is pushed sideways, because both sides are treated as carrying current the same way
- using Fleming's right hand rule, which belongs to the generator effect, and getting the rotation backwards
- pointing the first finger from south to north, when field lines run north to south outside the magnet
- claiming the turning effect is the same in every position, when it falls to zero once the coil is at right angles to the field
- describing a slip ring rather than a split ring, which would let the coil come to rest after half a turn
- saying the commutator reverses the magnetic field, when it reverses the current in the coil
- reversing both the current and the magnets and then predicting reversed rotation, when the two changes cancel
- calling the coil an electromagnet that is attracted to the magnet, instead of using the motor effect force
- writing a list of parts instead of a sequence of causes, which caps a levels answer at the lower band
- saying the magnets attract and repel the coil, which is the electromagnet idea rather than the motor effect
- putting the commutator reversal at the wrong moment, so that it cancels the rotation instead of maintaining it
- using Fleming's right hand rule, which belongs to the generator effect