Mains electricity

  • Created by: Holly45
  • Created on: 28-03-15 13:43

Power and energy change

  • power = current X potential difference
  • when an elecrtical chaarge goes though a charge in potential difference then energy is transfered
  • energy is supplied to the charge to 'raise' it though a potential
  • the charge gives up this energy when it 'falls' though any potential component elsewhere in the circuit
  • energy transferred = charge X potential difference
  • the bigger the change in P.D, the more energy is transferred for a given amount passing trough a circuit
  • it means that a battery with a bigger voltage will supply more energy to a circuit for every coulomb of charge which flows
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Energy and power in circuits

  • energy which supplies electricity is also supplying energy
  • whenever a current flows though anything with electrical resiustance then electrical energy is converted into heat energy
  • the more current that flows, the more heat is produced
  • a bigger voltage means more heatng becuase it pushed more current though
  • filament bubs work by passing a current though a very thin wire, heating until it glows
  • an efficient applience wastes energy
  • they transfer more of their total electrical energy output to useful energy
  • energy transferred = power rating X time
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Fuses and earthing

  • if a fault develops in which the live wire touches the metal case, then the case is earthed
  • this surge in current melts the fuse when the amount of current is greater than the fuse rating
  • this will cut off the live supply and break the circuit
    • this also isolates the appliance becuase it makes it impossible to get an electric shock and prevents fires
  • anything with double insulation doesn't need an earth wire
  • cables that carry the live and neutral wires are known as two-core cables
  • circuit breakers are an electrical safty devise
  • when they detect a surge in current they'll open a switch which breaks the circuit
  • they're more convenient than fuses
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Electricity in the home

  • most likely hazards are frayed cables, water near sockets, damaged plugs and too many plugs in one socket
  • electrical appliances are connected to the mains supply by three core cables
  • the brown LIVE WIRE alternates between a high negative and positive voltage
  • blue neutral wire is always at 0 volts
  • the earth wire is green and yellow and is for safety
  • no bare wires are showing in a plug
  • thicker cables have less resistance so they carry more current
  • metals parts are brass and copper becuase they're good conductors
  • cable grips are rubber becuase they're good insulators
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Mains electricity

  • mains supply is AC
  • UK mains supply is aprox 230v
  • the current is constantly changing direction
  • the frequency is 50Hz
  • cells and batteries supply direct current which keeps the current flowing in the same direction
  • if you plug an AC supply into an oscilloscope you get a 'trace' on the screen
  • if you plug in the DC supply you'll get a straight line
  • in an AC current, the height of the trace shows the potential difference
  • for DC the potential difference is the distance from the straight line to the center
  • frequency = 1 (divided by) time periods (s)
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