Electromagnetic induction

• What is induced when a wire is moved in a magnetic field?

  Current is induced.
• What happens to electrons in a conductor when it is moved in a magnetic field?

  Electrons move to one side of the conductor as the field changes.
• What occurs if the conductor is connected in a circuit?

  A current will flow.
• What does the current produced in a circuit generate?

  It produces its own magnetic field.
• How does the direction of the new magnetic field relate to the original magnetic field?

  The direction of the new field is in the opposite direction to the first field.
• What is the principle behind small-scale current production using a coil of wire and magnets?

  • Spinning a coil of wire between two permanent magnets induces a current.
  • A sensitive ammeter can show the current generated (only milliamps).
  • Passing a wire through a magnetic field also shows a deflection in an ammeter.
• Describe the process of large-scale current production in a thermal power station.

  1. Water heats up and evaporates to form steam (via combustion of fossil fuels or nuclear fission).
  2. Steam is put under pressure and forced into a turbine.
  3. The turbine rotates, connected to a massive coil of wire in a strong magnetic field (the generator).
  4. Current is generated in the coil by the spinning motion through the magnetic field.
• What factors affect the size of the current or voltage produced in electromagnetic induction?

  • Number of coils of wire
  • Speed of rotation
  • Magnetic field strength
• What happens to the current direction in an alternator every half turn?

  The current switches direction.
• Which rule is used to show the direction of the current induced in an alternator?

  The Left Hand Rule.
• What type of current does an alternator produce?

  Alternating current (AC).

  
• What is the role of the commutator in a dynamo?

  It reverses the sign of the current that flows from the coil.
• What type of current does a dynamo produce?

  Direct current (DC).
• How do microphones and loudspeakers work in relation to current and sound signals?

  • Microphones produce a current proportional to the sound signal.
  • A fixed magnet is at the center, with a coil of wire around it that moves due to pressure variations in sound waves.
  • The movement induces current in the coil.
  • Loudspeakers operate similarly, with current flowing into the coil causing movement and producing sound.
• What is the function of transformers in electrical systems?

  • AC in the first coil creates a changing magnetic field.
  • This changing magnetic field induces a current in the secondary coil.
  • If the primary current is DC, no current is induced in the secondary coil.
  • More coils on the secondary create a step-up transformer; fewer coils create a step-down transformer.
• What is the relationship between the number of coils on the primary and secondary in a transformer?


  \(\frac{N_{\text{primary}}}{N_{\text{secondary}}} = \frac{V_{\text{primary}}}{V_{\text{secondary}}}\)
• What is the purpose of the National Grid in electrical energy transmission?

  • Electrical energy is transferred at high voltages from power stations.
  • In domestic uses, energy is transformed to lower voltages for safety and efficiency.
  • High currents lead to greater heating effects in wires, causing energy loss.
  • Increasing voltage reduces current, minimizing energy loss.
  • Voltage is decreased near towns for safety.
• What do step-up and step-down transformers do?

  • Step-up transformers increase voltage.
  • Step-down transformers decrease voltage.
• What is the power relationship for a transformer with 100% efficiency?

  \(V_{\text{primary}} \times I_{\text{primary}} = V_{\text{secondary}} \times I_{\text{secondary}}\)
• What is an alternator?

  A device that makes use of the generator effect to generate alternating current.
• What is a dynamo?

  A device that makes use of the generator effect to generate direct current.
• What is the generator effect?

  When there is relative motion between an electrical conductor and a magnetic field, a potential difference is induced across the ends of the conductor.
• What happens if the conductor in the generator effect is part of a complete circuit?

  A current will flow through the conductor.
• What is the function of a loudspeaker?

  A device that converts variations in current into vibrations of a diaphragm to produce corresponding sound waves.
• What is the function of a microphone?

  A device that converts the pressure variations in sound into corresponding current variations.
• Why are power cables used in the National Grid?

  They transport electricity at very high voltages to reduce energy loss and make transportation more efficient.
• What is a step-down transformer?

  A transformer that has a smaller potential difference in the secondary coil than in the primary coil due to having fewer turns in the secondary coil.
• What is a step-up transformer?

  A transformer that has a larger potential difference in the secondary coil than in the primary coil due to having more turns in the secondary coil.
• What is the purpose of a transformer?

  To change the magnitude of an alternating voltage using an iron core with primary and secondary coils of wire.
• What is the turns ratio in a transformer?

  The number of turns in the primary coil over the number of turns in the secondary coil.
• What does the turns ratio equal in a 100% efficient transformer?

  It is equal to the voltage ratio.
• What is the voltage ratio in a transformer?

  The potential difference across the primary coil over the potential difference across the secondary coil.