P3 - Supplying Electricity

• Direct current (d.c.)

  Current provided by a cell that only flows in one direction
• Alternating current (a.c.)

  Current that repeatedly reverses direction
• Mains electricity is supplied as alternating current
• Mains electricity has a frequency of 50Hz and a voltage of 230V
• Colours of wires in a three-core cable

  • Live - brown
  • Neutral - blue
  • Earth - green and yellow stripes
• Function of the live wire

  Carries the alternating potential difference from the supply
• Function of the neutral wire

  Completes the circuit
• Function of the earth wire

  Safety wire to stop the appliance becoming live when there is a fault
• There is a current in the earth wire when there is a fault
• Why the live wire is dangerous

  Provides a large potential difference that would cause a large current to flow through a person if they touched it
• National Grid

  Nationwide network of cables and transformers that link power stations to customers
• Step-up transformers in the National Grid

  Increase the potential difference from the power station to the transmission cables
• Step-down transformers in the National Grid

  Decrease the potential difference from the transmission cables to the mains supply in buildings so that it is safe to use
• Having a large potential difference in the transmission cables

  Helps to make the National Grid an efficient way to transfer energy by requiring a small current to transfer the same amount of power, reducing electrical power wasted due to heating
• Units

  • Power - watt (W)
  • Current - amp (A)
  • Potential difference - volt (V)
  • Resistance - ohm (Ω)
• What two things does energy transfer to an appliance depend on?

  Power of appliance, time it is switched on for