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# Thread: Physics Problem of the Week!

1. 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.

2. 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.

3. 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...

4. Originally Posted by masterazn
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...
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.

5. 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.
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.

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

7. 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.

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

9. 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.

10. Originally Posted by zarikdon
To recap quickly, there are 3 laws of thermodynamics.
You left out the zeroth law

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