Physics Problem of the Week!

[masterazn]

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?
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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?

[Corneo]

Then again since when did warm sea water existed?

[Anakha1]

Then again since when did warm sea water existed?

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

[asdf]

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

[masterazn]

asdf, could you explain?

[asdf]

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.

[masterazn]

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

[Rewen]

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

Note: Physics nube. :xeek:

[zarikdon]

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

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.

[Avalon]

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...