7.5.5

# Electromagnetic Induction

Test yourself

## 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.

### 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.

The two main laws about electromagnetic induction are from Faraday and Lenz.

• 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
• $emf=-N\frac{{\Delta\phi}}{{\Delta}t}$

### 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.