Physics - P2.5 - Mains Electricity

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P2.5.1 - Alternating Current

  • Direct current: cells/battery supply - current passes in one direction
  • Alternating current (ac): mains supply - passes in one direction, reverses and passes in other direction
  • Frequency of UK mains supply = 50Hz - changes direction 50 times/second, voltage of mains = 230V
  • Live wire of mains supply alternates between positive and negative potential with respect to neutral wire (which stays at 0V)
  • Live wire alternates between peak voltages of +325V and -325V - equvilent to direct pd of 230V
  • Frequency of ac supply determined from oscilloscope trace using:
  • f = 1 / T
    • f - frequency of ac - Hz
    • T - time for one cycle - s
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P2.5.2 - Cables and Plugs

  • Most electrical appliances connected to sockets of mains supply using cable and three-pin plug
  • Outer cover made of plastic or rubber: insulators, plug pins made of brass: conductor, hard, won't rust or oxidise
  • Important cable grip fastened tightly over cable - should be no bare wires showing and correct cable must be connected firmly to pin
  • Brown wire: live pin, blue wire: neutral pin, green and yellow wire: earth pin
  • Earth wire - only in three-core cable, must have one for appliances with metal cases: case attatched to earth wire in cable, connected to longest pin, appliances with plastic cases don't need to be earthed: double-insulated and connected with two-core cable
  • Different thickness cable = different purpose - more current = thicker
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P2.5.3 - Fuses

  • Plastic cased appliances don't need to be earthed: insulated - metal cased appliances are earthed: fault occurs and live wire touches case, case becomes live and may give someone a shock
  • Fuse - fitted in series with live wire: cuts appliances off if it blows - fault occurs in earthed appliance: large current flows to earth and melts fuse
  • Fuse rating should be slightly higher than normal working current: too high = fuse won't melt soon enough, too low = fuse will melt when turned on
  • Circuit breaker - can replace fuse - electromagnetic switch that opens and cuts of supply if current exceeds certain value
  • Residual current circuit breaker (RCCB) - cuts off current in live wire if different to neutral wire current - faster than fuse or normal circuit breaker
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P2.5.4 - Electrical Power and Potential Difference

  • Appliances transfer electrical energy other energy forms - rate: power
  • P = E / t
    • P - power - W
    • E - energy transferred - J
    • t - time - s
  • More usual to measure current through appliance and pd across than energy transferred and time in electrical circuits
  • P = I X V (used to calculate current in appliance: work out fuse needed)
    • P - power - W
    • I - current - A
    • V - potential difference - V
  • Power rating shown on electrical appliances - pd of mains: 230V - fuse size used is slightly higher than fuse value calcualted
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P2.5.5 - Electrical Energy and Charge

  • Electrical current: rate of flow of charge
  • Q = I X t
    • Q - charge - C
    • I - current - A
    • t - time - s
  • Electrical energy changed to other forms when charge flows through appliance
  • In resistor, electrical energy transferred to resistor: resistor becomes hotter
  • E = V X Q
    • E - energy - J
    • V - potential difference - V
    • Q - charge - C
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P2.5.6 - Electrical Issues

  • Electrical faults may occur due to damaged sockets, plugs, cables or appliances: equipment checked regularly for wear - worn/damaged items replaced/repaired by qualified electrician
  • Aviod overloading sockets - cause overheating: risk of fire
  • Safely handle appliances and keep away from water
  • Cable shoud be appropriate for use
  • Electrical appliance should be chosen according to power, efficiency rating and cost
  • Filament and halogen bulbs less efficient and don't last as long as low-energy bulbs - number of different low-energy bulbs avaliable
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