Edexcel IGCSE Physics · Spec 6.4-6.5
The Motor Effect & the d.c. Motor
The motor effect and how a d.c. motor works.
Physics revision video
The Motor Effect & the d.c. Motor
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Explained
The motor effect and the d.c. motor
Put a wire carrying a current into a magnetic field and the wire is pushed. That is the motor effect. It happens because the current creates its own magnetic field around the wire, and the two fields interact, producing a force that shoves the wire out of the way.
Fleming's left hand rule
Hold the thumb, first finger and second finger of your left hand at right angles to each other. The First finger points along the Field, from north to south. The seCond finger points along the Current, from positive to negative. The thuMb then gives the direction of the Motion, which is the force.
Left hand for motors. If either the current or the field is reversed, the force reverses. If both are reversed, the force stays in the same direction, which is a favourite question.
The force is greatest when the wire is at right angles to the field, and zero when the wire lies along the field.
Making it turn
A single wire is pushed in one direction and that is all. A motor uses a coil, so the two sides of the coil carry current in opposite directions. The field is the same for both, so by the left hand rule one side is pushed up and the other is pushed down. Two forces in opposite directions on opposite sides of a pivot produce a turning effect, and the coil rotates.
Why a commutator is needed
Half a turn later, the sides of the coil have swapped places. Without intervention the same forces would now push it back the other way and the coil would rock rather than spin.
The split ring commutator reverses the current direction every half turn, so the force on each side reverses at exactly the moment the sides swap. The turning effect therefore always acts the same way round, and rotation continues. The brushes press against the commutator to keep the electrical contact while it turns.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Physics record that many candidates knew there was a magnetic field around the coil which interacted with the permanent field, and scored two marks for it. The marks beyond that were harder to reach.
Two reasons are given. Fewer candidates referred to the force on the individual wires, describing only the force on the coil as a whole, which meant later mark points were not available to them. And some assumed there was no interaction or force when the coil was vertical, which is not correct.
So write about the two sides of the coil separately. Say which way each is pushed, and say that the two opposite forces produce a turning effect.
Making a motor turn faster
Increase the current, use a stronger magnet, or add more turns to the coil. All three increase the size of the force, and therefore the turning effect.
Spec 6.4-6.5
What you need to know
- Describe the motor effect
- Use Fleming's left-hand rule
- Explain how a d.c. motor turns
Active recall
Quick check
Answer each question before opening the answer.
What is the motor effect?
A current-carrying wire placed in a magnetic field experiences a force.
How can you reverse the direction of the force on the wire?
Reverse the direction of the current or reverse the magnetic field.
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