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Clarification About Waste Heat

  1. Dec 9, 2008 #1
    I've never been too clear on some aspects of waste heat. I know the law of conservation of energy says that the amount of energy must remain constant, but my confusion comes from exactly what form it ends up taking. As an example if I swing a hammer and hit something I know that chemical energy is transformed to kinetic energy in my arm, which is then transformed to heat in the object I hit, as well as the sound of it being hit, and the work of deforming the object slightly. The question I have though is does all that energy end up simply becoming heat? I know that sound is just movement of air, so it would seem logical that it would become heat through friction, is that so? Then as for the actual work of deforming the object where does the energy that went into doing that go? Is that in fact where the heat came from?

    To give another example, say I moved some weight over a horizontal distance, where does the energy I burn go? Is it all transferred to heat? Is it the same heat that warms my muscles? It seems like the energy that escapes the inefficiencies of my body and actually does the work couldn't then warm my muscles, so the question then is where does that energy go?

    One last example which is different from the previous ones. In a car when you are in neutral is more heat produced in the engine since no work is being done? I assume the engine burns a constant amount of gas at a certain rpms since the same chemical energy is being turned into heat, but without then doing any work more must remain as heat in the engine right? Then if the car is in gear and in motion I assume that extra heat is being turned into kinetic energy and then being lost to friction and drag, and thus becoming heat again.

    I guess in summery what I'm asking is does all energy end up as heat, or is there another form of waste energy?
     
  2. jcsd
  3. Dec 9, 2008 #2

    russ_watters

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    Welcome to PF.
    Almost all of the energy in that case becomes heat. When you permanently deform a metal, some of the energy will be absorbed by the changing configuration of the molecular bonds. Everything else becomes heat.
    The friction between the weight and the ground dissipates energy as heat.
    Not more, but all is wasted, so 100% waste or 0% efficiency.
    Actually, the work depends on the fuel/air mixture also, so it isn't necessarily going to be constant at constant rpm. I'm not quite following the last part, but when at idle, the energy is dissipated in basically four ways: friction inside the engine and drivetrain (up to and including the clutch), the radiator, the alternator, and the tailpipe (ok, and maybe the heat/AC too). Together, these account for all of the energy of the engine.
    Yes, on flat ground. If you drive up a hill, some is converted to potential energy. Probably should also point out that if you charge the battery, you store some that way...
    Almos all of it, but there is sometimes changes in chemical bond energy and gravitational potential energy.
     
  4. Dec 9, 2008 #3
    Thank you for your detailed reply. The changes in chemical bonds was what I was overlooking. I would like to make sure I understand your answer for where the energy goes when moving a weight. You said:
    Does this mean that with each step I would push down on the ground and that would create some heat there, which would be equivalent to the energy I was using to do the work (minus inefficiencies)?

    Thanks again for clearing this up for me.
     
  5. Dec 9, 2008 #4

    russ_watters

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    You didn't say if you were carrying or sliding the object, so I assumed sliding. When you carry an object, there is zero work being done on the object. There is also zero work being done by your feet when you walk on a horizontal surface (assuming negligible air resistance). All of the energy that is dissipated is dissipated in making your muscles function and accelerating and decelerating your body.
     
  6. Dec 9, 2008 #5
    Try to play around with an air hockey table, to get an idea of how important friction is (by seeing what changes when you get rid of it).

    That assumption is very wrong - the rate at which you must feed gas to the engine depends on both the rpm and the load on the engine.

    But in general I think you're on the right track - eventually, in most everyday processes, energy ends up as heat via friction.
     
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