Course: Motors: Combined + Separate | loudspeakers and generator effect: Separate only | Tier: Higher only | Linked equation: F = BIl
Notes
Electric motors (C+S, HT)
A coil of wire carrying a current in a magnetic field tends to rotate: the current flows in opposite directions on the two sides of the coil, so the forces on them act in opposite directions. A split-ring commutator reverses the current every half turn, so the coil keeps rotating the same way. The motor turns faster with a bigger current, a stronger magnet or more turns on the coil.
Loudspeakers (S only, HT)
Loudspeakers and headphones use the motor effect. An alternating current in a coil, which sits in the field of a permanent magnet, produces a force that keeps changing direction. This makes the coil and cone vibrate, producing pressure variations in the air – sound waves.
The generator effect (S only, HT)
- A potential difference is induced across a conductor when it moves relative to a magnetic field, or when the magnetic field around it changes. If the conductor is part of a complete circuit, a current flows.
- The induced current produces its own magnetic field that opposes the original change.
- To increase the induced p.d.: move faster, use a stronger magnet, or use more turns on the coil.
- Alternator: a rotating coil with slip rings produces a.c. Dynamo: uses a split-ring commutator to produce d.c.
- Microphones use the generator effect: sound waves make a diaphragm and coil vibrate in a magnetic field, inducing an alternating p.d. that matches the sound.
- Transformers also use this effect – see the transformer pages.
Questions
Q1 (C+S) Why does a current-carrying coil in a magnetic field rotate?
Show answer
The current flows in opposite directions on each side of the coil, so the forces on the two sides act in opposite directions (one up, one down). This turns the coil.
Q2 (C+S) What is the job of the split-ring commutator in a motor?
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It reverses the direction of the current in the coil every half turn, so the coil keeps rotating in the same direction.
Q3 (C+S) Give two ways to make a motor spin faster.
Show answer
Any two of: increase the current; use a stronger magnet; add more turns to the coil.
Q4 (S only) Explain how a loudspeaker produces sound.
Show answer
An alternating current flows through a coil in the field of a permanent magnet. The force on the coil keeps changing direction (motor effect), so the coil and cone vibrate back and forth. This creates pressure variations in the air – sound waves.
Q5 (S only) A magnet is pushed into a coil connected to an ammeter. Describe what the ammeter shows when the magnet is pushed in, held still, and pulled out.
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Pushed in: a current flows (the needle moves one way). Held still: no current – the field isn’t changing. Pulled out: a current flows in the opposite direction.
Q6 (S only) Give three ways to increase the p.d. induced by a generator.
Show answer
Rotate the coil faster; use a stronger magnet; use more turns on the coil.
Q7 (S only) What is the difference between an alternator and a dynamo?
Show answer
An alternator uses slip rings and produces alternating current (a.c.). A dynamo uses a split-ring commutator and produces direct current (d.c.).
Q8 (S only) Explain how a moving-coil microphone turns sound into an electrical signal.
Show answer
Sound waves make the diaphragm vibrate, moving the attached coil back and forth in a magnetic field. This induces an alternating p.d. across the coil (generator effect) that varies in the same way as the sound waves.
