How a loudspeaker uses the motor effect, then the reverse: moving a magnet and a coil induces a potential difference, what makes it bigger, and which way the current flows.
What you will learn
- Explain how a loudspeaker or headphones make sound (Physics only)
- Say when a potential difference is induced and what makes it bigger (Higher)
- Predict the direction of an induced current from the change that caused it (Higher)
The loudspeaker
A loudspeaker is the motor effect at work.
A light cone is fixed to a coil that sits in the field of a permanent magnet.
- The amplifier sends an alternating current through the coil
- The coil is in a magnetic field, so it feels a force
- Each time the current reverses, the force reverses: the coil moves in, then out
- The cone moves with it and pushes the air: pressure waves, which we hear as sound
The cone vibrates at the frequency of the current, so a higher frequency gives a higher pitch, and a bigger current gives bigger vibrations: a louder sound. Headphones are tiny loudspeakers.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- The loudspeaker
- Running it backwards (Higher)
- Which way does it flow? (Higher)
- Try it: Inside the loudspeaker
9 practice questions, marked as you go; about 25 minutes in all.
Key terms
Cone, Coil, Permanent magnet, Potential difference is induced, Generator effect, Cuts field lines, Changes, Faster, Stronger, More turns
In this unit: Magnetism and electromagnetism
Magnets and field lines, electromagnets, the motor effect and F = BIl, then loudspeakers, generators, microphones and transformers.
- Magnets and magnetic fields
- Electromagnetism
- The motor effect
- Loudspeakers and induced potential difference
- Alternators, dynamos and microphones
- Transformers