Electromagnetic Induction

• 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

  
• For a fixed conductor in a changing magnetic field

  • 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
• Fixed conductor in a changing magnetic field

  A) magnetic field

  B) wire

  C) ammeter

  D) current

  E) induced

  F) circuit

  
• 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 Law - 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
• 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

  A) towards coil

  B) north pole

  
• Explanation of why a magnet will become a north pole if pushed near the coil:

  • 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
• 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

  A) pulled

  B) south pole

  
• Explanation of why a magnet will become south pole when pulled away from the coil:

  • 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
• 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

  • 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
• Remember that current is always in the direction of positive charge carriers. Therefore, current flows from the positive to the negative terminal of the battery
• EMF induced in a conductor is proportional to the rate at which the magnetic field lines are cut by the conductors
• You can increase magnetic field lines by increasing strength of the magnets

  • more lines = crossing at higher rate = more EMF
• You can increase lines to be cut by increasing number of wires
• Electromagnet induction - when a wire is moved across a magnetic field, a small EMF is generated in the wire

  • EMF induced in the wire
• Faraday’s law - the induced voltage/EMF (& the current) can be increased by:

  • moving a wire faster
  • using a stronger magnet
  • making the wire into a coil and pushing the magnet in or out of the coil
• If the wire in electromagnetic induction is put in a complete circuit, the induced EMF makes an electric current pass around the circuit
• Electromagnet opposes what you do:

  • move north in → becomes north
  • move south in → becomes south
  • move north out → becomes south
  • move south out → becomes north
• Lenz’s law - the direction of the induced current is such that it opposes the change producing it
• Current passes aNticlockwise or couNterclockwise round the North pole end
• Current passes clockwise round the South pole end
• • Rule for the polarity of a coil

  A) anticlockwise

  B) North

  C) clockwise

  D) south

  
• Lenz’s law’s applications:

  • sticking magnet into solonoid makes it repel it
  • taking magnet away from the solonoid causes it to attract it
  • current changes direction
• Lenz’s law comes from the idea of conservation of energy

  • energy isn’t created or destroyed