Demonstrating Induction (Cambridge (CIE) IGCSE Physics): Revision Note

Demonstrating induction

• Electromagnetic induction is used in:

  • electrical generators which convert mechanical energy to electrical energy

  • transformers which are used in electrical power transmission

• The phenomenon of electromagnetic induction can be demonstrated using

  • a magnet and a coil

  • a wire and a U-shaped magnet

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:

  1. When the bar magnet is stationary, 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

     • As a result, no e.m.f. is induced in the coil

  2. When the bar magnet is moved inside the coil, there is a reading on the voltmeter

     • As the bar magnet moves, its magnetic field lines are cut by the coil

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

  3. When the bar magnet is moved back out of the coil, there is a reading on the voltmeter with the opposite sign

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

     • An e.m.f. is induced in the opposite direction, shown momentarily by the reading on the voltmeter with the opposite sign

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

• Factors that will increase 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 magnet

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

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

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

• The expected results are:

  1. When the wire is stationary, the voltmeter shows a zero reading

     • When there is no relative motion between the wire and the magnetic field, no field lines are cut

     • As a result, no e.m.f. is induced in the wire

  2. As the wire is moved between the magnetic poles, there is a reading on the voltmeter

     • As the wire moves, it cuts the magnetic field lines of the magnet

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

  3. When the wire is moved back out of the magnet, there is a reading on the voltmeter with the opposite sign

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

     • An e.m.f. is induced in the opposite direction, shown momentarily by the reading on the voltmeter with the opposite sign

• 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 electromagnetic induction

• There are certain factors that can affect the magnitude and direction of induced e.m.f.

Factors affecting the magnitude of the induced e.m.f.

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 size of the induced e.m.f.

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 size of the induced e.m.f.

   • This is because each turn (loop) of wire in the coil cuts the magnetic field lines 

   • Therefore, the total induced e.m.f. increases with each additional turn (loop)

1. The size of the coils:

   • Increasing the area of the coils will increase the size of the induced e.m.f.

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

2. The strength of the magnetic field:

   • Increasing the strength of the magnetic field will increase the size of the induced e.m.f.

   • This is because there will be more magnetic field lines in a given area

Factors affecting the direction of the induced e.m.f.

1. The orientation of the poles of the magnet:

   • Switching the poles of the magnet induces an e.m.f. in the opposite direction

2. The direction in which the wire, coil or magnet is moved:

   • Reversing the direction in which the wire, coil or magnet is moved induces an e.m.f. in the opposite direction