Demonstrating Induction (Cambridge (CIE) O Level Physics): Revision Note

Demonstrating Induction

• An EMF can be induced either when:

  • A conductor, such as a wire, 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

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 EMF

A bar magnet is moved through a coil connected to a voltmeter to induce an EMF

The expected results are:

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

  • When the bar magnet is held still inside, or outside, the coil, there is no cutting of magnetic field lines, so, there is no EMF 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

  • This induces an EMF 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 (a result of Lenz's law)

  • 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

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

  • This is Lenz's law

Lenz's Law

An e.m.f is induced only when the bar magnet is moving through the coil

• Factors that will increase the induced EMF 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 magnet

• When a long wire is connected to a voltmeter and moved between two magnets, an EMF is induced

• The pattern of a magnetic field in a wire can be investigated using this set up

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

A wire is moved between two magnets connected to a voltmeter to induce an EMF

The expected results are:

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

  • When the wire is held still inside, or outside, the magnets, the rate of change of flux is zero, so, there is no EMF induced

• As the wire is moved through between the magnets, an EMF 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 EMF 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 induced e.m.f are:

  • Increasing the length of the wire

  • Moving the wire between the magnets faster

  • Increasing the strength of the magnets

Factors Affecting EM Induction

• The magnitude (size) of the induced EMF is determined by:

  • The speed at which the wire, coil or magnet is moved

  • The number of turns on the coils of wire

  • The size of the coils

  • The strength of the magnetic field

• The direction of the induced potential difference is determined by:

  • The orientation of the poles of the magnet

1. The speed at which the wire, coil or magnet is moved:

• Increasing the speed will increase the rate at which the magnetic field lines are cut

• This will increase the induced potential difference

2. The number of turns on the coils in the wire:

• Increasing the number of turns on the coils in the wire will increase the potential difference induced

• This is because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines

3. The size of the coils:

• Increasing the area of the coils will increase the potential difference induced

• This is because there will be more wire to cut through the magnetic field lines

4. The strength of the magnetic field:

• Increasing the strength of the magnetic field will increase the potential difference induced

5. The orientation of the poles of the magnet:

• Reversing the direction in which the wire, coil or magnet is moved