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Energy transfers - electric kettle

Discussion in 'Science' started by gumgamble, Sep 17, 2019.

  1. gumgamble

    gumgamble New commenter

    Hi all,

    Our department is trying to get our heads around teaching energy using the correct terminology. As non of us are physics specialists I would really appreciate your help!

    What are the energy stores and transfers in an electric kettle? It’s on the specification but we all disagree on what we think they are. Is it;

    Electrostatic store - electrical transfer - internal energy

    Or is it heat transfer? Or perhaps it’s more complicated?

    Any advice would be appreciated.

    Thanks
     
  2. gainly

    gainly Lead commenter

    I really don't like the stuff about energy stores in the new specification. But I think it would be sufficient to say:
    Energy is transferred electrically to the thermal energy store of the heating element. The element transfers energy by heating to the thermal energy store of the water.
     
  3. BW12345

    BW12345 Lead commenter

    The element is not an energy store, at all.

    You could start with an energy store such as a hydroelectric dam - the store is in the potential energy of the water.
    As the falling water loses potential energy it gains kinetic energy, which is then used to generate electricity. A voltage is generated, which we see as the mains, 230 Volts AC.
    Electricity can't be stored as such, except in capacitors (don't go there, perhaps). To store, convert 'lectric to chemicals (Batteries) , or sometimes in a spinning wheel (Swiss buses, or underground at Harwell in Oxford).

    The kettle element is an electrical conductor, so there's electric current flowing through it. It's not a very good conductor, so it has what's called Resistance. As the elecrons are forced through the element by the voltage, they transfer energy to the resistance wire, all coiled up inside the kettle element, which gets hot. The energy is "dissipated" there. That energy is carried away by thermal conduction, to the water, which heats up.

    For the potential energy of the water in the dam, use m.g.h.
    I doubt you'll want to look at Volts and Amps but Volts x Amps = Watts.
    Show them a kettle which may say 3000W on the label. That's 3000 Joules per second. Multipy by the time in seconds it takes to heat a litre of water from 20° to 100° and you know how much energy you used.
    Then you can work out how much water had to fall, how far, in the dam to create enough energy to boil one litre.

    You're then storing the energy as heat in the water which will be dissipated to the room, or your insides if you drink it.


    Why does the resistance of the element heat up? I suggest an analogy - something like electrons losing energy as they bounce about colliding with the metal atoms, or even "friction". If you get into the superpositions of quantum states you could be there a while...


    The kettle element is somewhat analogous to a conventional water-filled radiator in a house, which they could relate to better, perhaps. Yes radiators do store some hot water, but only so it can lose heat to the air around them. You'd need a double one about 2m long by 600 high to dissipate 3kw energy into the room. The kettle element is far more space-efficient, and it has a negligible energy storage capacity.
    Those horrible old "Radiant" electric fires are better for explanations, but kids have never seen them now!
     
    Last edited: Sep 17, 2019
    bonxie likes this.
  4. BW12345

    BW12345 Lead commenter

    (I wish I could edit a few words there!) and add:
    fire.jpg
     

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