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    A kettle is 90% efficient.
    To boil water 420 kJ is needed.
    Calculate how much electrical energy is used by the kettle.

    energy = ................................ ......

    Suggest reasons why the kettle is only 90% efficient.
    ................................ ................................ ................................ ................................ ................................ ...
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    For the equation simply refer to your textbook for a proper explanation.

    For the reason that it is only 90percent efficient, try and think what the ten percent of the energy is being converted into, which will be waste energy such as...

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    Thermal energy and sound energy,
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    So if the kettle is 90% efficient and a total of 420kJ is inputted, you need to find what the useful output is (as kettles need electrical energy, anything else is waste). So 420*0.9 would be how much of the total 420kJ is the useful output (which is electrical).
    As for reasons for the 90%, you need to think about what is happening to the energy that isn't been turned into electrical energy. The most obvious one would be loss of thermal energy in the wire (before it reaches the heating element) due to the resistance in the wire. You might say sound energy too but that may not be correct as the question is asking about the electrical energy, and it will only be converted into sound energy after it's been turned into thermal energy (as in a kettle it goes electrical -> thermal, it's converted to sound instead of thermal not in the process of becoming heated, if that makes any sense).
 
 
 
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