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

Induced EMF & Lenz's Law

• An EMF will be induced in a conductor if there is relative movement between the conductor and the magnetic field

• It will also be induced if the conductor is stationary in a changing magnetic field

• For an electrical conductor moving in a fixed magnetic field

  • The conductor (e.g wire) cuts through the fields lines

  • This induces an EMF in the wire

Current-Carrying Wire Moving Through a Magnetic Field

Moving an electrical conductor in a magnetic field to induce an EMF

Bar Magnet Moving Through a Solenoid

When the magnet enters the coil, the field lines cut through the turns, inducing an EMF

• For a fixed conductor in a changing magnetic filed

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

  • This induces an EMF in the coil

A magnet moved towards a wire creates a changing magnetic field and induces a current in the wire

• A sensitive voltmeter can be used to measure the size of the induced EMF

• 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

• Lenz Law states:

  The direction of an induced potential difference always opposes the change that produces it

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

Demonstrating Lenz's Law

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

• Explanation

  • Due to the generator effect, a potential difference will be induced in the coil

  • The induced potential difference always opposes the change that produces it

  • The coil will apply a force to oppose the magnet being pushed into the coil

  • Therefore, the end of the coil closest to the magnet will become a north pole

  • This means it will repel the north pole of the magnet

Bar Magnet Approaching a Solenoid

Magnet being pushed into a coil of wire inducing a current in the wire

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

• Explanation:

  • Due to the generator effect, a potential difference will be induced in the coil

  • The induced potential difference always opposes the change that produces it

  • The coil will apply a force to oppose the magnet being pulled away from the coil

  • Therefore, the end of the coil closest to the magnet will become a south pole

  • This means it will attract the north pole of the magnet

Bar Magnet Leaving Solenoid

Magnet being pulled away from a coil of wire due to force of attraction

Right-Hand Dynamo Rule

• When moving a wire through a magnetic field, the direction of the induced EMF can be worked out by using the Right-Hand Dynamo rule

 Right-Hand Dynamo Rule

The Right-Hand Dynamo rule can be used to deduce the direction of the induced EMF

• To use the 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 direction of the induced EMF always opposes the change that produces it

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