Electromagnetic Induction
Physics: AQA A Level, topic 7.5.5
Electromagnetic Induction
A current-carrying wire in a magnetic field may experience a force. In reverse, a potential difference (and hence a current) may be induced in a conductor that experiences a change in magnetic flux.
Slow falling magnet
- In this demonstration, the magnet falling through the tube takes considerably longer to fall than it would if it were just falling through the air.
Explanation
- The copper tube is not magnetic, but it is a conductor experiencing a change in magnetic flux.
- A current is induced in the tube (in a circle around the magnet), which in turn causes a magnetic field to act so as to slow the magnet down.
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Wire moving in magnetic field
- As the wire cuts through the magnetic field lines, a p.d. and hence a current is induced in the complete circuit. This is registered as a small flicker on the ammeter.
Faraday and Lenz's Laws
The two main laws about electromagnetic induction are from Faraday and Lenz.
Faraday's law
- When the magnetic flux linkage in a circuit changes, an electromotive force (emf) is induced in the circuit.
- The emf is proportional to the rate of change of the flux linkage. The equation for calculating the emf is:
- emf = −(number of coils x change in flux linkage) ÷ change in time
Lenz's law
- The negative sign at the start of Faraday's law is because of Lenz's law.
- Lenz's law is that the induced electromotive force will induce a current and a magnetic field which will oppose the change in flux.
Test yourself on Electromagnetic Induction
Which of these statements are true about electromagnetic induction?
What is the correct form of Faraday's law?