First Law: "The internal energy of a system is constant unless it is changed by doing work or by heating."
The first law of thermodynamics is often summarized as 'conservation of energy'.
Heat and work are equivalent ways of changing a system's energy.
The above sentence needs a context to give it meaning. If you
have a machine performing work, and it is giving off heat, that
heat is the net difference between the ideal output of work and
the actual output of work.
You may calculate that you are supposed to get 100 kilowatts
of work from a machine, but you only get 80 kilowatts because you
are losing 20 kilowatts worth of energy in the form of heat.
Second Law: "For any spontaneous
process, there is always an increase in the entropy of the universe."
Entropy is defined by the symbol S, and has the units of J K-1.
Entropy is hard to define. At times, the word appears to
be the chemist's synonym for disorder.
An crystal of sodium chloride has a degree of order that
is lost when the crystal dissolves in water. The spontaneous
character of sodium chloride dissolving in water is attributed
to the increase of entropy as the order present in the
crystal lattice is lost.
You might ask "Well, if dissolving sodium chloride is
spontaneous, why is it that when the water from a sodium
chloride solution evaporates, the sodium chloride
lattice reforms?
The textbook answer: "The entropy increase corresponding
to the transformation of water liquid to water vapor exceeds
the entropy decrease corresponding to the formation of
sodium chloride solid from aqueous sodium chloride
ions."
Third Law: "The entropy of a pure
crystalline substance at absolute zero is zero."
S(0 K) = 0
We could reach zero entropy if we could reach absolute zero.
Can we reach absolute zero?
No.
Imagine that you have the coldest object on the planet. How
do you make it colder?
An object is magnetized, causing a loss of entropy.
The object is then allowed to demagnetize, and the process
causes a loss of heat.
This is illustrated by the graph below.
In the graph below the line for the entropy of the magnetized state (blue) is lower than the line for the unmagnetized state (purple).
White arrows moving down:
Magnetization occurs.
Entropy decreases.
Temperature remains constant.
White arrows moving to the left:
Demagnetization occurs.
Entropy remains constant.
Temperature decreases.
As the temperature approaches absolute zero, the difference
in entropy between the magnetized and the unmagnetized states
decreases. Since the cooling is proportional to this difference, the cooling effect with each change becomes less
and less.
The cooling rate decreases.
There is a heating rate that should be included in the
math.
When the cooling rate (which is decreasing) becomes
equal to the heating rate, no further cooling will occur.
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