P5 - Electricity in the Home

• Direct current (d.c.)


  • Current only flows in one direction
  • Produced by a direct potential difference
  • Provided by cells or batteries
• Alternating current (a.c.)


  • Repeatedly reverses direction
  • Produced by an alternating potential difference
  • Provided by mains electricity
• The positive and negative terminals of an alternating power supply swap over with a regular frequency
• The frequency of the mains electricity supply in the UK is 50 Hz and its voltage is 230 V
• The Earth wire


  • Green and yellow
  • Safety wire to stop the applicance becoming live
  • The potential difference is 0V
  • Only carries a current if there is a fault
• The neutral wire


  • Blue
  • Completes the circuit
  • Potential difference of 0V
• The live wire


  • Brown
  • Dangerous due to the high potential difference (230V)
• Fuse


  • Connected to the live wire
  • If the live wire touches the neutral wire, a largee current flows called a short circuit
  • When this happens, the fuse melts and disconnects the wire from the mains, keeping the appliance safe
• Three core cable


  • What most appliances in the UK use
  • Copper is used for the wires because it is a good conductor and bends easily
• Plastic is used for the wire coatings and plug case because it is a good insulator
• The National Grid


  A nationwide network of cables and transformers that link power stations to homes, offices, and other consumers of mains electricity
• Transformers


  Devices that can change the potential difference of an alternating current
• The process of the National Grid

  • Power stations generate elecricity at an alternating potential difference of about 25,000 V
  • Step-up transformers are used to increase the potential difference from the power station to the transformer cables
  • The cables in the National Grid transfer electrical power at a potential difference of up to 400,000 V
  • Step-down transformers are used to decrease the potential difference from the transmission cables to the mains supply in homes and offices so that it is safe to use
  • Homes and offices use electrical power supplied at a potential difference of 230 V
• Transformers improve efficiency because a higher potential difference across the transmission cables means that a lower current is needed to transfer the same amount of power
• power (W) = current (A) x potential difference (V)
• • A lower current in cables means that less power is wasted due to heating of the cables
  • This is because the power lost in heating a cable is power (W) = current^2 (A) x resistance (ohms)
  • This makes the National Grid an efficient way to transfer energy
• charge flow (C) = current (A) x time (s)
• energy transferred (J) = charge flow (C) x potential difference (V)
• power (W) = energy transferred (J) / time (s)