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Jeremy Double · April 23rd 11, 10:56 AM posted to uk.transport.london,uk.railway

On 23/04/2011 11:20, d wrote:
On Sat, 23 Apr 2011 11:12:36 +0100
Jeremy wrote:
Heat will spontaneously flow from a high temperature to a low
temperature, but not in the other direction (this is the "Zeroth law of
thermodynamics").

If you want to create ice when the ambient temperature is above 0 deg C,
then work is needed to pump the heat "uphill" from the water you are
freezing to the surroundings. That's what the motor does in a
refrigerator. The work used to pump the heat need not come from a heat
source itself, it could come from hydro power, for instance.

But the work done by water expanding as it freezes will be considerably
less than the work needed to freeze the water in the first place.

I'm not talking about in a fridge - I'm talking about water sitting outside
on a cold night with the heat disappearing off into space. No energy is
being used to remove the heat yet the water freezes and the ice can do work
on anything nearby as it expands.

If what is being claimed is true - ie that heat is always required to do
work then this should be impossible.

Heat isn't always required to do work _directly_: hydro-power uses the
potential energy of water flowing downhill to perform work, for
instance. But in that case the energy that moved the water from the
seas to the hills to allow it to perform work came from the heat of the sun.

If work is performed using heat as the energy source, then the laws of
thermodynamics apply, and there is a limit to the amount of work that
can be got out of a certain quantity of heat, which can be calculated
from the ratio of the absolute temperature of the heat source to the
absolute temperature of the heat sink.

Clearly it isn't so that rule is wrong
in this instance just as it doesn't apply inside a car cylinder which was
my initial statement.

You originally wrote:

Internal combustion engines don't convert heat to work. The work is done
by the pressure of the gas from the chemical reaction. Heat is a useless
byproduct of this reaction that has to be got rid of.

Internal combustion engines _are_ a type of heat engine, they do convert
heat to work. The laws of thermodynamics apply to them just as they do
to a steam engine or steam turbine. The only difference is that the
heat is created inside the cylinder, not outside.

The fuel burns in the air charge, converting chemical energy to (mostly)
heat and a small amount of work. The heat causes the gases to expand,
allowing them to do work and move the piston. Almost all of the
expansion of the combustion gases is due to the heat liberated by
combustion, not due to the increased number of moles of gas (for
instance, 1 mole of carbon burning uses 1 mole of oxygen to give 1 mole
of carbon dioxide). Remember that most of the gas in the cylinder of an
engine is nitrogen from the charge air (air is about 79% nitrogen).

You are right that there is some useless heat created in combustion,
which has to be got rid of. That is a consequence of the second law of
thermodynamics, which says that there is a limit to the amount of heat
that can be converted to work. The useful work depends on the ratio of
the temperatures of the heat source (i.e. the combustion temperature of
the fuel) to the heat sink (i.e. the temperature of the exhaust gas).

If you need to understand this better, I suggest you read the book I quoted.
--
Jeremy Double {real address, include nospam}
Rail and transport photos at
http://www.flickr.com/photos/jmdoubl...7603834894248/