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Basic question about energy conservation

  1. Feb 21, 2014 #1

    I just read this http://io9.com/scientists-just-created-some-of-the-most-powerful-muscl-1526957560 where they explain a new type of revolutionary artificial muscle.

    The principle of operation is very simple: when a material gets hotter, it contracts and generates a force. I already knew materials can expand or contract when their temperature vary, but it looks like i havent thought about it deeply enough.

    If I have a thin bar of a metal tied to the roof, and 1 ton of iron tied to the bar. Then, I spend 1 watt of energy to cool the bar, and it contracts by 1 cm. This contraction lifts the ton of iron, increasing its potential energy by much more than 1 watt. Loos like I'm creating energy, which of course its not possible, but that's my question: where is that energy coming from?

    Best regards
  2. jcsd
  3. Feb 21, 2014 #2


    Staff: Mentor

    Hi mcolom, welcome to PF!

    There is no extra energy in this system. These materials are loaded under tension in order to form the artificial muscle. That loading under tension requires a substantial amount of energy. The heat doesn't provide much, if any, of the energy. It merely triggers the release of the energy that is already stored.
  4. Feb 21, 2014 #3
    Did you actually run any calculations using the real mechanical and thermal properties of iron to evaluate this? My guess is that you are just pulling numbers out of the air. When you do run the calculations, you will find that energy is not being created. One other issue: the watt is a unit of power, not energy.
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