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