Answers to Using Electricity

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  1. a)Two wires. The live (brown) and neutral (blue).
    b)The earth wire is missing because the appliance is double insulated, ie the casing is made of plastic.The symbol shown indicates this fact.
    c) Use the formula P = V x I. Stick in the numbers to get...
    1000 = 230 x I
    1000/230 = I
    So I = 4.3 A
    d) Use a 5 A fuse .
    e) Use the formula Q = I x t
    Q = 4.3 x ( 3 x 60 ) remember time has to be in seconds!
    Q = 774 Coulombs.
  2. The diagrams below show how to do the measuring:
  3. The ammeter reads 2A. This is because the current in the branches of a parallel circuit must add up to the amount of current leaving the battery.
  4. The voltage across the cell must read 9V. Remember that the sum of the voltages across each component in a series circuit must add up to give the cell or battery voltage.
  5. Use the formula P = V x I to give P = 6 x 0.006 = 0.036W
  6. The diagram below shows the values:
  7. We can use the formula: P = I2R
    So sticking in the numbers 2,000 = 102 x R
    2,000/100 = R
    20 ohms.
  8. Sarah could build a continuity circuit as shown below:
    Touching one end of the wire to the earth pin of the plug and the other to the metal casing will light the bulb if the electricity can flow from the casing to the earth pin.
    This indicates that the kettle is earthed properly.
    If the bulb did not light then this indicates that the earth wire is not connected to the metal casing and this could prove fatal if the live wire from the kettle`s element breaks and touches the kettle`s metal casing.

    Please don`t try and play with the mains electrics. Leave this to a qualified electrician...OK!


  9. In a ring circuit the current to a particular appliance splits up, check diagram below. Because it splits this means that if a particilar appliance draws 10A then the current in the cables will be only 5A. This means that thinner cables can be used since for large currents thicker cables are needed. Also there is less curent flowing in the cables, so the cables don`t get as warm.
  10. You can do this in two steps.. use P = V x I to find the current.
    Then use the current and voltage to find R using V = I x R.
    P = V x I
    950 = 230 x I, so 950/230 = I......This gives I = 4.1A.
    Now put 4.1A and 230V into the formula V = I x R
    230 = 4.1 x R.........This gives 230/4.1 = R.
    R = 56.1 ohms.
    Alternatively you can use the formula P = V2 / R.
  11. Use the formula Energy = Power X Time
    E = P x T
    E = 950 x ( 2 x 60 ) Watch out, time is in seconds
    E = 114,000J
  12. Use the formula V = I x R. V = voltage, I = current & R = resistance.
    So plug in the numbers
    V = I x R
    12 = I x 2,000
    12 / 2,000 = 0.006 A or 6mA
  13. The unit kWh is a unit of Energy not power.
    1 kWh means an appliance rated at 1kW is on for 1 hour.
    We can calculate the energy the appliance uses by the formula E = P x t
    Watch out time must be in seconds...1 hour = 60mins = (60 x 60 )seconds
    = 3,600 seconds. So plug this time into the equation to get
    E = 1,000 W x 3,600 seconds
    E = 3,600,000 J
    So 1kwh = 3,600,000J or 3.6 x 10 6 J ... 3.6 MJ if you want to be fancy!
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