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Physics Problem of the Week!

Discussion in 'Off-Topic' started by masterazn, Jan 28, 2004. | Replies: 23 | Views: 754

  1. masterazn

    masterazn Banned

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    Physics Problem of the Week!

    A ship heats its boilers and propels itself without the use of coal or oil in the following way: it pumps in warm sea water, concentrates the extracted heat in its boilers; and discharges the cooled sea water (possibly as ice) back into the ocean.

    Does it violate the conservation of energy act?

    Will this work?
    --------------------
    this is a problem for school, it's not worth anything really.

    So far, I've said this does not violate the conservation of energy act because the heat is taken from the water and used to heat the boilers. But I think this will not work because there won't be enough heat from the water (plus, if it worked, we would be doing that now).

    What are you ideas? Am I correct?
  2. Corneo

    Corneo IncGamers Member

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    Then again since when did warm sea water existed?
  3. Anakha1

    Anakha1 Banned

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    You've never been to the carribean, I take it.
  4. asdf

    asdf IncGamers Member

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    it violates the second law of thermodynamics, but not the laws of energy conservation...

    edit: i believe it's actually the second... too lazy to open a textbook
  5. masterazn

    masterazn Banned

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    asdf, could you explain?
  6. asdf

    asdf IncGamers Member

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    well, in a nutshell, the energy is conserved so that rule isn't broken

    well, it's more like an extention of the second law, which is essentially that total entropy will increase, and won't decrease without a certain amount of energy put into the system.

    the upshot of all that is, you have to do work (generating heat in the process since there is no known 100% efficient heat exchange machine known to man) to get some extra heat out of the system.
  7. masterazn

    masterazn Banned

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    i don't understand....could you explain it from some kind of beggining?
  8. Rewen

    Rewen IncGamers Member

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    Hmm by what method would the "heat" by "extracted" and through what type of mechanism? Or is this purely hypothetical?

    Note: Physics nube. :xeek:
  9. zarikdon

    zarikdon IncGamers Member

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    To understand the question, you probably have to know a little thermodynamics, which was what asdf was getting at with the 2nd law of thermo. Thermodynamics should probably be more properly called "statistical mechanics," but all that means is that it concerns itself with the mechanics (motion, energy, stuff like that) of a "statistically large" number of particles. Like what happens to the individual water molecules when you boil a pot of water. Statistically large here just means that it's much much larger than 1.

    To recap quickly, there are 3 laws of thermodynamics.

    1) Energy conservation.

    2) If you have an engine that takes an energy input, it can't convert ALL of that energy into useful work. Essentially, it means that there is no 100% efficient engine. An alternative way to state this is that the entropy of a system will always increase unless you actively pump energy into it. Entropy can be roughly thought of as the amount of disorder in whatever you're looking at.

    3) At zero temperature, all materials have the universal property that their entropy is zero.

    These three laws are all independent of each other... and your question really only revolves around the first two. Or really, only the second one, so let's ignore the third one for now. You're right that energy is conserved, so the boat's engine doesn't violate the first law. However, it does violate the second law. The problem occurs when you say that the engine "condenses the heat" from the water. Extracting the heat from the water is actually decreasing the disorder (entropy) of the water, and requires you to put IN energy. In fact, you need to put in more energy into your engine than the amount of energy you're getting out. That's the basic idea of the 2nd law.
  10. Avalon

    Avalon IncGamers Member

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    Hey, the bike's the most effiicient input-to-output ration machine we've got right?
    Somewhere in the area of 60%?

    Master, is that exactly how the problem is stated?
    I'dve thought the coal/oil boilers heat water in to steam, which in turn drives sets of stem turbines, which then turn the drive shaft or propellers. This water, can I'd assume come from the outside environ...and be returned once it cools back in to liquid form...
  11. Underseer

    Underseer IncGamers Member

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    I have a background in physics, but it seems the practical answer is more appropriate here.

    With current technology, there is no way to contruct a heat engine as you suggest. If you wanted to suck in sea water, suck the heat out of it then dump the resulting ice and/or cold sea water overboard, you would lose energy, not make it. In all likelihood, the way to acheive this would be through refridgeration and anyone who's ever paid an electric bill knows refridgerators lose energy, not make it.
  12. Anakha1

    Anakha1 Banned

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    He's talking about using the heat from the water to power engines that spit the same water, now cold, back out to propel the ship. Theoretically I think it would be possible, but I don't think we have the technology, nor would the water provide enough heat naturally.

    But then, I know jack all about physics beyond when you enable gravity by hurling your little sister's barbie off the top story window into a wood chipper when you're nine, mom gets pretty pissy.
  13. masterazn

    masterazn Banned

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    well, actually, the problem is states exactly liek that. I'm assuming the teacher means the water is extracted of heat and the heat is used to heat a boiler which then drives a turbine to a propeller.

    let me get this straight. so it DOES violate the conservation of energy act because you need to put in energy to get out energy of the water? I'm very confused...
  14. tarnok

    tarnok IncGamers Member

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    No, it _doesn't_ violate conservation of energy. It is impossible as far as modern physics is concerned because it requires the total entropy of the system to decrease which is supposed to be impossible. So it checks out under conservation of energy, but fails because of another law of thermodynamics.

    As an interesting side-note, I read an article a few months ago in a waiting room somewhere that claimed that scientists had found decreases in entropy in very small scale closed systems. Don't ask me what magazine it was, I can't recall.
  15. Underseer

    Underseer IncGamers Member

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    I know what he's talking about and as I said, that's not possible. Sucking the heat energy out of the water would cost more energy than you would get.

    Heat engines only work if you have a temperature difference. However with a boat on an ocean, you're not going to have a temperature difference unless you create one. Since no process is 100% energy efficient (ain't no such thing as a closed system), creating a temperature difference will cost more energy than you'll get out of it when you use the resulting temperature difference to drive some sort of heat engine.
  16. masterazn

    masterazn Banned

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    hrm okay, this was one weird problem....i kinda get it. but why does it require heat to extract heat!?
  17. Underseer

    Underseer IncGamers Member

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    Creating a temperature difference requires work. Think about it. If you can use a temperature difference to do work (wihch is what a heat engine is), then it must cost energy to create a temperature difference, right?

    Think about a refridgerator. Those things are about as efficient as technology allows. They create a temperature difference by moving heat from the interior of the refridgerator to the coils in back.
  18. masterazn

    masterazn Banned

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    well...oh i see, but wouldn't it work if you had enough heat?
  19. Crispyknight

    Crispyknight IncGamers Member

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    No. Unless I'm sadly mistaken in my understanding of physics, (possible!) you will never have a temperature gradient like the one in the problem under passive conditions. You'd have to be pumping energy into the system to do this, and it would be more efficient to just put the energy into the boilers and go. You could get propulsion on a boiling ocean, but thats not gonna happen anythie soon.
  20. maccool

    maccool IncGamers Member

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    You left out the zeroth law ;)

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