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Various ways of doing work to minimize entropy increase

  1. Jul 22, 2015 #1
    [Moderator's note: Recategorized thread to "Basic".]

    While driving alone through the beautiful scenery of Banff and Yoho national parks, a question formed in my mind.

    Which of these modes of slowing down a vehicle by an equal amount is likely to minimize the resulting overall increase in entropy? I’m assuming that the reduction in kinetic energy can be accomplished in a less or more efficient manner.

    1) Applying the brakes.
    2) Opening all the windows.
    3) Turning on the AC compressor, and letting the turning of the wheels power it.
    4) Downshifting into a lower gear, and using engine braking.
    5) Applying a rainfall.
    6) Applying a magnetic field.

    Any other methods that can be suggested are welcome, if they make any difference to the outcome.

    A number of reasonable assumptions would need to be made about the specifics involved, and that’s fine. I realize that there may be too many unknowns and not enough specifics, or that the answer may very well be ‘same’ for all the modes involved.

    I get a sense that this is either a really easy problem, or a really hard one.
    Last edited by a moderator: Jul 22, 2015
  2. jcsd
  3. Jul 22, 2015 #2


    Staff: Mentor

    I don't think this question is answerable as you ask it; there are too many unknowns, and all of the methods you suggest involve a lot of dissipation and hence will probably involve a large and not easily estimated increase in entropy.

    A more interesting comparison would be between any of the methods you suggest--but #1 is probably the best since it's at least the usual method for a conventional vehicle--and the following:

    7) Use regenerative braking to convert the vehicle's kinetic energy to stored energy in some storage device (the usual one in a hybrid vehicle is a battery).
  4. Jul 22, 2015 #3
    Technically, that would be a cooler version of #6, right?

    That should obviously be better in the long run, since some of what would become entropy is stored as usable energy, right? Since it actually improves fuel economy, whereas all of the above should have the same effect.
  5. Jul 22, 2015 #4


    Staff: Mentor

    No. Applying a magnetic field to the car as a whole is not at all the same as regenerative braking. As far as the car as a whole is concerned, regenerative braking is just braking; it applies mechanical force at the wheels to slow the car down. The only way magnetic fields come into it is the magnetic fields inside the motor/generator that is recapturing the energy and putting it into the battery.

    Basically, yes, although the way you've put this is not quite right. The correct way to put it would be that some of what would become unusable energy is stored as usable energy. Energy does not "become entropy". Entropy increases when the amount of unusable energy increases and the amount of usable energy decreases, but energy and entropy are still distinct things.
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