topic 13 electromagnetic induction

• What is electromagnetic induction?

  the induction of a potential difference in a wire which is experiencing a change in magnetic field
• Where can EM induction occur?

  • magnet moving into a coil of wire
  • coil of wire rotates in a magnetic field
• A changing magnetic field induces a potential difference in a conductor
• How is an alternating current produced?

  By moving the magnet back and forth OR rotating in the same direction
• Steps

  • relative motion between magnet and wire
  • wire experiences a change in magnetic field
  • induced potential difference across wire
  • if complete circuit, induced current flows in wire
• When magnetic field through an electrical conductor changes, there is an induced potential difference
• How to increase induced current?

  • move magnet/coil faster
  • use stronger magnet
  • use more coils
• What happens if there is no relative movement?

  there will be no electromagnetic induction and no induced potential difference produced across
• When current is induced in a wire, that current produces its own magnetic field
• Direction of movement

  the wire and magnetic field must move perpendicular to each other
• What happens if the wire and magnetic field move parallel?

  no current is induced
• Lenz's Law

  the direction of induced current is always such as to oppose the change that causes the current
• What are generators?

  use electromagnetic induction to induce a current and converts kinetic energy into electrical energy
• What do generators consist of?

  A coil of wire rotating inside a magnetic field
• What happens as the coil in a generator turns?

  A voltage is induced in the wire
• What applies a force to rotate a coil in a magnetic field?

  The generator's mechanism
• What occurs as the coil spins in a generator?

  Current is induced in the coil
• How does the current behave in a generator?

  The current changes direction every half turn
• What are dynamos classified as?

  D.C. generators
• What component allows a dynamo to produce direct current?

  A split-ring commutator
• What does the split-ring commutator do in a dynamo?

  Swaps the connection every half turn
• How does the commutator affect the current in a dynamo?

  Keeps the current flowing in the same direction
• What do alternators use instead of split-ring commutators?

  Slip rings and brushes
• What is the role of brushes in an alternator?

  Ensure contacts don’t swap every half turn
• What type of current does an alternator produce?

  Alternating potential difference
• How do microphones convert sound waves?

  Into variations in current in electrical circuits
• What happens when sound waves hit the diaphragm in a microphone?

  The diaphragm vibrates, generating current
• What does the movement of the coil in a microphone depend on?

  The properties of the sound waves
• How do loudspeakers function?

  Convert variations in electrical current into sound waves
• What causes the cone in a loudspeaker to move?

  The force from the AC signal on the coil
• What replaces the diaphragm in a loudspeaker?

  A paper cone
• What happens when the current is reversed in a loudspeaker?

  The force acts in the opposite direction
• How do the movements of the cone in a loudspeaker create sound waves?

  They cause variations in air pressure
• What do transformers use?

  they use induction to change the size of the potential difference of an alternating current
• What do transformers include?

  it has two coils of wire, primary and secondary coils joined with a soft iron core
• Secondary coil

  not connected directly to the primary side or any power supply

  • works using electromagnetic induction
• When is a current induced in a coil in a transformer?

  when magnetic field is perpendicular to secondary coil and changes a current is induced in the coil
• Is potential difference bigger in primary or secondary coil?

  greater in secondary coil because it has more turns
• Do transformers use AC or DC?

  cannot be DC because there will be no change in magnetic field and no induced potential difference/current will form in the secondary coil

  • AC must be used
• Transformers are almost 100% efficient, input power = output power