Distinction should be made between the
energy terms heat and work. Both represent
energy in transition.
Work is the transfer of energy resulting from a force acting
through a distance. Heat
is energy transferred as the result of a temperature
difference. Neither heat nor work are thermodynamic
properties of a system. Heat can be transferred into or out
of a system and work can
be done on or by a system, but a system cannot contain or
store either heat or work. Heat into
a system and work out of a system are considered positive
quantities.
When a
temperature difference exists across a boundary, the Second
Law of Thermodynamics indicates
the natural flow of energy is from the hotter body to the
colder body. The Second Law of
Thermodynamics denies the possibility of ever completely
converting into work all the heat supplied
to a system operating in a cycle. The Second Law of
Thermodynamics, described by Max
Planck in 1903, states that:
It is impossible to construct an engine that
will work in a complete cycle and produce
no other effect except the raising of a weight and the
cooling of a reservoir.
The second
law says that if you draw heat from a reservoir to raise a
weight, lowering the weight will
not generate enough heat to return the reservoir to its
original temperature, and eventually the
cycle will stop. If two blocks of metal at different
temperatures are thermally insulated from their
surroundings and are brought into contact with each other the
heat will flow from the hotter to
the colder. Eventually the two blocks will reach the same
temperature, and heat transfer will cease.
Energy has not been lost, but instead some energy has been
transferred from one block to
another.
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