Topic 13

• Electromagnetic induction


  When an electrical conductor moves relative to a magnetic field, a voltage (potential difference) is induced across the conductor
• Electromagnetic induction


  When there is a change to the magnetic field that an electrical conductor is placed in, the voltage (potential difference) is also changed
• Producing an electric current using a magnet and a conductor on a small-scale

  1. Moving a coil of wire between magnetic poles

  2. Moving a magnet within a coil of wire
• Once a voltage is induced, if there is a complete circuit, a current will also be induced
• Producing electrical current on a large-scale


  An electromagnet is rotated around a coil
• Factors affecting the size of an induced voltage

  • The number of turns on the coil of wire
  • How strong the magnetic field is
  • The speed of the movement
• Requirement for an induced potential difference to cause a current flow

  The conductor must form a closed loop or be part of a complete circuit
• Direction of the magnetic field produced by an induced current

  • The direction of the field is such that it opposes the change that induced the current
  • The change is either the movement of the conductor or a change to the field
• Ways the generator effect is used to generate different types of current

  • In an alternator to produce alternating-current
  • In a dynamo to produce direct-current
• How a dynamo generates current

  1. A coil of wire rotates inside a magnetic field

  2. A commutator is used to ensure it continues rotating in the same direction, therefore keeping the current flowing in the same direction (d.c. current)
• How electromagnetic induction is used in alternators to generate alternating current

  1. A coil of wire rotates in a magnetic field

  2. The end of this coil is connected to slip rings which will cause the current to change direction while rotating, producing a.c.
• Electromagnetic effect a microphone takes advantage of

  The generator effect
• How a microphone works


  It converts the pressure variations in sound waves into alternating current
• How loudspeakers make use of the motor effect

  The motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produce audible sound
• How a loudspeaker works

  1. A cone wrapped in wire is connected to an a.c power supply and is placed in a permanent magnetic field

  2. When current flows through the wire, it creates a magnetic field which interacts with the permanent field

  3. This produces a force which causes the cone to vibrate, producing sound
• How the pitch of the sound from a loudspeaker is changed

  • The frequency of the a.c current is altered
  • This creates a different frequency of vibration in the cone
• How microwaves convert sound into electrical signals

  1. Sound produces pressure variations which cause the diaphragm to vibrate

  2. A coil of wire is connected to the diaphragm, so as the diaphragm moves, the coil also moves

  3. Movement in a magnetic field = a p.d. is induced

  4. As there is a complete circuit a current is also created
• Transformer


  A primary coil and a secondary coil of wire wrapped around an iron core
• Iron core


  • It is easily magnetised
• How a transformer works

  1. An alternating current flows through the primary coil

  2. This induces a changing magnetic field in the core

  3. This changing magnetic field induces an emf, which produces a current in the secondary coil
• Why the current in the primary coil must be alternating


  • For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing
  • For the magnetic field to be changing, the current in the primary coil must be alternating
• For a 100% efficient transformer, the electrical power input is equal to the electrical power output
• Equation linking number of turns with p.d. in transformers


  primary p.d. / secondary p.d. = primary turns / secondary turns
• Power equation for transformers with 100% efficiency (using voltage and current)
• Step-down transformer


  The secondary coil has fewer turns
• Advantage of a step-down transformer in the national grid

  It means that the voltage can be reduced to a value safe enough to be used in houses
• Importance of step-up transformers in the national grid

  • It can increase efficiency
  • For the same power, a higher voltage will lead to a lower current (P=VxI)
  • The lower the current, the less energy that is lost
  • Used for transmission across power lines
• How a step-up transformer works

  1. The primary coil has fewer turns

  2. An a.c. current produced a magnetic field in the primary coil

  3. The iron core passes the magnetic field to the secondary coil

  4. The magnetic field induces a higher voltage in the secondary coil

  5. Power is conserved, so this produces a lower current
• Where step-up and step-down transformers are used in the national grid

  Step-up transformers are used at power stations, whilst step-down transformers are used domestically
• Explain how an electromagnetic induction (generator effect) is used in dynamos.


  A coil of wire is turned inside a magnetic field, which induces a magnetic field, and hence current, in the coil of wire. The coil is connected to a commutator which switches the connections every half-turn. This causes the induced current in the external circuit to be in one direction (d.c.).