Page 52 - 4498

heat is rejected to a cold reservoir at the temperature T and the rest of the
2
energy is converted into work.
We can get useful work from thermal energy only if we have a heat source
that is at the temperature T greater than that of its surroundings. Kelvin
y
and Klausius formalized this idea in a statement of what they called the
second law of thermodynamics.
1) There is no natural process the only result of which is to cool a heat

reservoir and do external work (Kelvin formulation).
2) Heat by itself flows only from the bodies at higher temperature to the
bodies at lower temperature (Klausius formulation).
The second law of thermodynamics states the following: heat can be

made to go from a body at lower temperature to the one at higher
temperature only if external work is done. In refrigerators we compel heat
to go from a colder to a warmer body, but work must be done to produce

this unnatural heat flow in accordance with the second law of
thermodynamics. This means that work and heat are a hot equivalent of
different forms of the energy transfer.

Although there is a vast store of thermal energy in the ocean, we cannot
simply take this thermal energy and convert it into work to operate a ship.
Only if we have some body at a lower temperature than the ocean to which

we can transfer the part of the energy can make an engine operate from the
ocean's heat energy. Even then we can convert into work only the
difference between the heat energy provided at the source and the heat
energy rejected to the colder reservoir.

On the other hand, we can always convert mechanical energy into heat
energy completely. Thus, the second law of thermodynamics indicates a
certain irreversibility of natural processes. For example, we can always

convert the kinetic energy of an automobile into heat in the brake drums
and tires, but we cannot reconvert all this heat energy to kinetic energy.
If we have 50 g of water at 0 °C and 50 g at 100 °C, we can always mix
them to obtain 100 g at 50 °C. But, if we have 100 g of water at 50 °C, we

cannot pour half of it into one vessel and have it at 0 °C and the other half
into a second vessel and have it at 100 °C. This would be the violation of
the second law of thermodynamics, which tells us that many natural

processes occur only in one direction.
At the end there's another formulation of the second law of
thermodynamics: a perpetual motion machine of the second kind is

impossible

52