GCSE P2 Chapter 5

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  • Created by: emma998
  • Created on: 09-03-14 20:13

Alternating current

  • Direct current = current that flows in one direction only, batteries use direct current
  • Alternating current = current that constantly changes direction fluctuates from positive to negative. Passes in one direction then reverses and passes in the other direction
  • Mains supply uses alternating(ac) current
  • Frequency of 50herz meaning it changes direction 50 times each second
  • Live wire of the mains supply alternates between positve and negative potential differences with respect to the neutral wire which stays at zero
  • Potential difference of +325V - -325V, 
  • Equivalent to a direct potential difference of +230V
  • Frequency of alternating current can be found using an oscilloscope
  • Frequency(Hz) = 1 / time taken for one cycle(seconds)
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Cables and plugs

  • Most electrical appliances are connected to the sockets of the mains supply by a cable and three-pin plug
  • The outer cover of a three-pin plug is made of plastic or rubber which are good electrical insulators
  • The pins of the plug are made of brass which is a good conductor electricity, hard and does not rust or oxidise
  • It is important that the cable grip is fastly tightened over the cable. There should be no bare wires showing and each cable must be firmly connected to the terminal of the correct pin
  • The brown wire is connected to the live pin and is called the live wire
  • The blue wire is connected to the neutral pin and is called the neutral wire
  • The green and yellow stripey wire (of a 3-core cable) is connected to the earth pin (the longest pin). A 2-core cable does not have an earth wire
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Mains appliances

  • Mains appliances with metal cases must be earthed:
    • If a fault develops and the live wire touches the metal casing
    • The casing could become live
    • Could give an electric shock to anyone who touches it
  • Appliances with plastic cases do not need to be earthed:
    • They are said to be doubly-insulated
    • If a fault develops and the live wire touches the casing
    • Casing cannot become live
    • Cannot give an electric shock to anyone who touches it
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Fuses

  • Fuses are fitted in series with the live wire between the live wire and the appliance so that if a fault develops the fuse will melt cutting the appliance off from the live wire
  • A fuse contains a thin piece of metal wire which easily melts if the current exceeds a specific value
  • There are different fuses which have different fuse ratings
  • The rating shows the maximum curent which can pass through the wire without the fuse melting
  • Circuit breakers are electromagnetic switches which can open disconnecting the supply if the current exceeds a specific value
  • Residual currency circuit breakers are even quicker than normal circuit breakers. They are designed to open disconnecting the supply if the current through the live wire differs from the current through the neutral wire
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Electrical power and potential difference

  • Power is the rate of transfer of energy
  • Power = energy / time
  • However in an elctric circuit it is more useful and common to measure current and potential difference
  • Power = Current X potential difference 
  • Electrical appliances all show their power rating
  • From this the current through the appliance can be worked out 
  • Current = Power rating / 230(mains supply)
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Electrical energy and charge

  • Charge = Current X time
  • Current is the rate of flow of charge
  • When an electrical current flows through an appliance, electrical energy is transferred into other forms of energy useful to the appliance
  • In a resistor electrical energy is transfered to the resistor so the resistor becomes hot
  • Energy(J) = Charge(C) X Potential difference(V)
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Electrical issues

  • An electrical fault may occur as a result of damage to sockets, plugs, cables and appliances
  • Electrical equipment should be checked regulary for wear
  • Worn or damaged items should be replaced or repaired by a qualified electrician
  • Avoid overloading sokcets because it can cause a large increase in current flowing through the wires causing overheating and causing the risk of a fire
  • Electrical appliances must be handled safely and never touched with wet hands or used in the bathroom
  • When choosing an electrical appliance power and efficiency rating should be considered as well as the cost
  • Low energy bulbs are much more energy efficient and last much longer than filament and halogen bulbs
  • There are a number of low-energy bulbs available
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