Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

If energy is never lost where is it?

  1. May 16, 2010 #1
    I know this seems like its a stupid question and I THINK I might know the solution but I want it certified.
    My teacher told me energy is never lost but where does it go?
    It turns into diffrent energy forms right?
    Well why do we not use this energy that is being lost into the air by say heat, why don´t we use it?
    I mean yes question is how but there must be a way to use this heat energy right?
  2. jcsd
  3. May 16, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Generally you don't use energy - you use the difference in energy.
    You can use waste heat from a process if you have something else colder to create a temperature difference and allow a heat flow.
  4. May 16, 2010 #3


    User Avatar
    Homework Helper

    There's also something called entropy which basically measures how free a system is to change itself given the amount of energy it has. It turns out that any physical process which decreases the total entropy of the universe can never occur. This is the Second Law of Thermodynamics.

    Turning waste heat into useful energy is one kind of process that results in a loss of entropy. (Think about it: in a hot gas, the molecules are free to move around in any direction, so entropy is high, but if the same gas is pushing a piston, the molecules have to be moving in such a way that they exert a force on the piston, so they have less freedom and less entropy. Roughly speaking anyway) That doesn't mean it's impossible to convert any of that heat into useful energy, but since it causes a loss in entropy, you have to pair your energy-extracting process up with some other process that increases the entropy enough to make up for it. Otherwise you'll wind up with an engine that doesn't work.
  5. May 16, 2010 #4


    User Avatar

    Staff: Mentor

    So to finish the thought, if your waste heat is released into the environment, it's gone and there is no further way to harness it across a temperature difference.

    Or, if you recover energy from the exhaust (of a car, for example), you recover it at a lower temperature than when the fuel initially burned it. So you gain less the second time than the first time you used the hot gases. You could do an endless number of heat recovery cycles, each time recovering less than the last.
  6. May 17, 2010 #5


    User Avatar
    Homework Helper

    There is light energy that goes beyond the outermost masses of the universe, which could be considered "lost" energy.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook