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

Why can't thermal energy be converted completely to motion

  1. Jul 11, 2011 #1
    In my thermo class I'm currently taking, the professor constantly says thermal energy cannot be completely converted into energy of motion. Just to be clear, I'm not arguing this point. I'm just wondering why we say that, when earlier in the course it said that thermal energy at the macroscopic level is the kinetic energy of the particles at the microscopic level. It seems to me that heat transfer is just transfer of kinetic energy... so what am I missing?
  2. jcsd
  3. Jul 11, 2011 #2


    User Avatar
    Science Advisor
    Gold Member

    I think what he means is that it can't be converted entirely into "usable" energy (e.g. to drive an engine). You can't get all the randomly moving particles to all of a sudden move in tandem to drive a machine because of the random nature of the particles. This is the second law of thermodynamics. There are not 100% efficient engines (where all the heat is converted to mechanical energy), there will always be excess heat left over moving into your cold reservoir from your hot reservoir.
  4. Jul 11, 2011 #3


    User Avatar

    Staff: Mentor

    Consider a hydroelectric dam. Can all of the stored energy drive a turbine? Why/why not?
  5. Jul 11, 2011 #4
    I see what you're saying russ, thanks for the response, but matterwave hit it on the head. I was just making sure I wasn't missing something important. Thanks again.

  6. Jul 12, 2011 #5
    If 100% of the thermal motion of a gas were converted to the kinetic energy of a piston, the gas would be left at absolute zero temperature, which is impossible. We can get very close to absolute zero, but never finally there because of quantum fluctuations.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook