Electromagnetic Induction (Cambridge (CIE) IGCSE Physics)

Induced e.m.f.

• An electromotive force (e.m.f.) is induced in a conductor whenever there is relative movement between the conductor and a magnetic field

• This could be when

  • the conductor moves in a stationary magnetic field

  • the conductor is stationary in a changing magnetic field

Induced e.m.f. due to a moving conductor

• For an electrical conductor moving in a fixed magnetic field:

  • the conductor (e.g. a wire) cuts the field lines

  • an e.m.f. is induced in the wire

When an electrical conductor moves in a magnetic field an e.m.f. is induced

Induced e.m.f. due to a moving field

• For a fixed conductor in a changing magnetic field:

  • as the magnet moves through the conductor (e.g. a coil), the field lines cut through the turns on the conductor (each individual wire)

  • an e.m.f. is induced in the coil

When a magnet is moved towards a wire, the changing magnetic field induces a current in the coil of wire

• A sensitive voltmeter can be used to measure the size of the induced e.m.f.

• If the conductor is part of a complete circuit then a current is induced in the conductor

  • This can be detected by an ammeter

Lenz's law

Extended tier only

• Lenz law states:

The direction of an induced emf always opposes the change causing it

• This means that any magnetic field created by an induced emf will act so that it tries to stop the wire or magnet from moving

Demonstrating Lenz's law

• Lenz's law can be demonstrated when a magnet is pushed into, or out of, a coil of wire

• If the magnet is pushed north end first into the coil, the end of the coil closest to the magnet will become a north pole

• This happens because:
  

  • the changing magnetic field induces an emf in the coil

  • the induced emf causes a current to flow and generates a magnetic field in the coil

  • the magnetic field due to the current opposes the magnet being pushed into the coil

  • therefore, the end of the coil closest to the magnet acts as a north pole

  • this means it repels the north pole of the magnet

When a magnet is pushed into a coil of wire, the end of the coil closest to the magnet will become a north pole and oppose its motion

• If a magnet is now pulled away from the coil of wire, the end of the coil closest to the magnet will become a south pole

• This happens because:
  

  • the changing magnetic field induces an emf in the coil

  • the induced emf causes a current to flow and generates a magnetic field in the coil

  • the magnetic field due to the current opposes the magnet being pulled away from the coil

  • therefore, the end of the coil closest to the magnet becomes a south pole

  • this means it attracts the north pole of the magnet

When a magnet is pulled away from a coil of wire, the end of the coil closest to the magnet will become a south pole and oppose its motion

Right-hand dynamo rule

Extended tier only

• When moving a wire through a magnetic field, the direction of the induced emf can be determined using the right-hand dynamo rule

• First Finger = Field:

  • Start by pointing the first finger (on the right hand) in the direction of the field

• ThuMb = Motion:

  • Next, point the thumb in the direction that the wire is moving in

• SeCond = Current:

  • The Second finger will now be pointing in the direction of the current (or, strictly speaking, the emf)

The right-hand dynamo rule can be used to deduce the direction of the induced emf