Principles of Electromagnetic Induction (AQA A Level Physics): Revision Note

Principles of electromagnetic induction

• Electromagnetic induction is a phenomenon which occurs when an e.m.f. is induced when a conductor moves through a magnetic field

• When the conductor cuts through the magnetic field lines:

  • This causes a change in magnetic flux ($∆\Phi$)

  • Which causes work to be done

  • This work is then transformed into electrical energy

• Therefore, if attached to a complete circuit, a current will be induced

• This is known as electromagnetic induction and is defined as:

The process by which an e.m.f. is induced in a circuit due to changes in magnetic flux

• This can occur either when:

  • a conductor cuts through a magnetic field

  • the direction of a magnetic field through a coil changes

• Electromagnetic induction is used in:

  • electrical generators, which convert mechanical energy to electrical energy

  • transformers, which are used in electrical power transmission

• This phenomenon can easily be demonstrated with a magnet and a coil, or a wire and two magnets

Experiment 1: Moving a magnet through a coil

• When a coil is connected to a sensitive voltmeter, a bar magnet can be moved in and out of the coil to induce an e.m.f.

The expected results are:

• When the bar magnet is not moving, the voltmeter shows a zero reading

  • When the bar magnet is stationary inside or outside the coil, the rate of change of flux is zero, so no e.m.f. is induced

• When the bar magnet begins to move inside the coil, there is a reading on the voltmeter

  • As the bar magnet moves, its magnetic field lines ‘cut through’ the coil, generating a change in magnetic flux (ΔΦ)

  • This induces an e.m.f within the coil, shown momentarily by the reading on the voltmeter

• When the bar magnet is taken back out of the coil, an e.m.f. is induced in the opposite direction

  • As the magnet changes direction, the direction of the current changes

  • The voltmeter will momentarily show a reading with the opposite sign

• Increasing the speed of the magnet induces an e.m.f. with a higher magnitude

  • As the speed of the magnet increases, the rate of change of flux increases

• The direction of the electric current, and e.m.f., induced in the conductor is such that it opposes the change that produces it

• Factors that will increase the magnitude of the induced e.m.f. are:

  • moving the magnet faster through the coil

  • adding more turns to the coil

  • increasing the strength of the bar magnet

Experiment 2: Moving a wire through a magnetic field

• When a long wire is connected to a voltmeter and moved between two magnets, an e.m.f. is induced

  • Note: there is no current flowing through the wire to start with

The expected results are:

• When the wire is not moving, the voltmeter shows a zero reading

  • When the wire is stationary inside or outside the magnet, the rate of change of flux is zero, so no e.m.f. is induced

• As the wire is moved through between the magnets, an e.m.f. is induced within the wire, shown momentarily by the reading on the voltmeter

  • As the wire moves, it ‘cuts through’ the magnetic field lines of the magnet, generating a change in magnetic flux

• When the wire is taken back out of the magnet, an e.m.f. is induced in the opposite direction

  • As the wire changes direction, the direction of the current changes

  • The voltmeter will momentarily show a reading with the opposite sign

• As before, the direction of the electric current, and e.m.f., induced in the conductor is such that it opposes the change that produces it

• Factors that will increase the magnitude of the induced e.m.f. are:

  • increasing the length of the wire in the field

  • moving the wire between the magnets faster

  • increasing the strength of the magnets