Thermodynamics Question?

Question

The laws of thermodynamics say that work cannot be obtained from a system by lowering its temperature below that of its surroundings. Engines, auto, steam convert large amounts of heat energy into useful mechanical work. However if you notice when you reduce the heat content of water and it freezes it expands with force strong enough to break a steel engine block. A force thru a distance equals work and I think the fractured block meets the definition of mechanical work. What goes here?

Answer 1

Quoting you: "... work cannot be obtained from a system by lowering its temperature below that of its surroundings ..."

The answer is really quite simple. You cannot freeze water if you cannot take the heat out of it. You cannot take the heat out of it if the water is colder than its surroundings. Heat only flows "down hill" which is to say from a hotter to a colder place.

In order to freeze water you will have to cool the surroundings to a temperature below the freezing point. If you do so you will not violate the rule you quote.

Answer 2

First of all, I'm confused which law of thermodynamics you are referring to in your question. Matter and energy within the system were conserved. Second, no energy entered of left the system.

The key element of your question is your definition of the 'system' involved. The surroundings of the engine block and the water it contains is what actually has the lower temperature. That is your 'system'. Keep in mind that a system is not necessarily the mechanical component, but can include standard atmosphere. When the block freezes you are not lowering it below the temperature of its surroundings, but rather keeping it at the same temperature. Also, where was the energy obtained to lower the temperature? From outside the system or as part of the system. No matter how you define the boundard, the laws remain intact.

Finally, the system had a certain potential energy, once the block is damaged, the potential energy of the system is lower. Once the water has fully expanded, it would no longer have the potential of doing work on the block.

Answer 3

It may be helpful to think about the thermodynamics of "heat engines" in terms of heat sources (flames?) and heat sinks (cooling water?). The engine does work only while conveying thermal energy from the source to the sink. In your case, the "source" may be the warm water in the "engine" and the sink may be cooling water capable of freezing the water in the engine. Because seawater or brine freezes below the temperature of frozen fresh water, you could use melting seawater or brine as your sink. Or you could use liquid nitrogen. The point is that the water in the enginge will not freeze and break the engine block unless heat flows from the engine to a sink at a lower temperature than the freezing point of the water in the engine. Incidentally, ships near the earth's poles must be concerned with the problem that cooling water taken from the sea can freeze fresh water systems within the ship.

Answer 4

It can, I suppose be argued as you have put forth, but I dare say you would be forcing the issue of "Work" obtained from a "System" with the simple freezing of water in a system. Water freezing is hardly "mechanical work" as water freezing does not meet the criteria of a "machine".

Answer 5

One could say that the water is in a sense acting like a wedge in the block. Instead of a force being applied to the wedge to split the block the force is derived from the phase state change of the water. Therefore we could say that the water is acting like a simple machine.

Answer 6

The expanding ice is making use of the latent heat of the water. Remember, a certain amount of energy is used at every phase change. That energy was simply harnessed in this case.

Answer 7

water freezes, expands and is harnessed inside a confined space, in this case the engine block, then continues to expand with enough energy to crack it. I suppose if you could find a usefull way to harness this energy you could call it work in this case you just have an expansion of liquid in a confined space