Electromagnetic Induction (OCR Gateway GCSE Physics)

Higher Tier Only

• Electromagnetic (EM) induction is used to generate electricity
• It is defined as:

A change in magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change

• This is done by the conductor or coil cutting through the magnetic field lines of the magnetic field
• This is often referred to as the generator effect and is the opposite to the motor effect
  • In the motor effect, there is already a current in the conductor which experiences a force
  • In the generator effect, there is no initial current in the conductor but one is induced (created) when it moves through a magnetic field

Generating Potential Difference

• A potential difference will be induced in the conductor if there is relative movement between the conductor and the magnetic field
• Moving the electrical conductor in a fixed magnetic field
  • When a conductor (such as a wire) is moved through a magnetic field, the wire cuts through the fields lines
  • This induces a potential difference in the wire

Moving an electrical conductor in a magnetic field to induce a potential difference

• Moving the magnetic field relative to a fixed conductor
  • As the magnet moved through the coil, the field lines cut through the turns on the coil
  • This induces a potential difference in the coil

When the magnet enters the coil, the field lines cut through the turns, inducing a potential difference

• A sensitive voltmeter can be used to measure the size of the induced potential difference
• If the conductor is part of a complete circuit then a current is induced in the conductor
  • This can be detected by an ammeter

Factors Affecting the Induced Potential Difference

• The size of the induced potential difference 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

The Magnetic Field Produced

• The direction of an induced potential difference always opposes the change that produces it
• This means that the magnetic field produced from EM induction also opposes the original change
  • The field will act so that it tries to stop the wire or magnet from moving

• 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

Magnet being pushed into a coil of 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

Magnet being pulled away from a coil of wire

ANSWER:  C

• • C is correct because there the magnet is stationary
  • This means there is no relative movement between the coil and the magnetic field, therefore there are no magnetic field lines being cut
  • If the magnetic field lines are not being cut then there will not be a potential difference induced

• • A, B & D are incorrect because a deflection on the voltmeter would indicate that a potential difference has been induced
  • This could only happen if there was relative movement between the coil and the magnetic field