Thermodynamics

economicsangriaMechanics

Oct 28, 2013 (3 years and 9 months ago)

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Thermodynamics is the
study of the effects of
work, heat and energy of a
system.

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It is primarily concerned
with the
macroscopic(large scale)
changes.

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All of thermodynamics can
be expressed in terms of
four quantities:

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Temperature (T)

*
Internal Energy (U)

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Heat (Q)

*
Entropy (S)

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The “
Zeroeth
” Law of
Thermodynamics

states that
if two systems are in thermal
equilibrium

with a third, then
they are in thermal
equilibrium with each other
(same temperature).


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British Scientist CP Snow states
the three laws humorously:

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1. You can’t win

*
2. You can’t break even

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3. You can’t get out of the
game

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energy is conserved for heat engines
working in a
cycle

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Q
= U
-

W, where Q is the net heat
into the engine, U is the increase in
internal energy, and W is the work
done by the engine.


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Work= Pressure times
the change in volume

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W=P
Δ
V or W=P(
V
f
-
V
i
)

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Work is positive when
gas expands

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Work is negative when
gas contracts

*
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Heat will not flow spontaneously from a cold
reservoir to hot reservoir
.

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Nor, can an engine or combination of engines
accomplish this
alone.

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No matter how efficient you are you will lose
energy.

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It
is not possible to convert 100% of heat into
work
.

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For an isolated system (and the Universe),
the entropy of heat flow is to increase
entropy.



*

*
*
Called Carnot efficiency, or the
ideal efficiency of a heat engine

*
(
T
hot
-
T
cold
)/
T
hot

= Ideal
efficiency

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Temperatures must be in
Kelvin.

*
Result is a percentage
.

*
*
absolute zero cannot be reached
.


*
But, experimenters try to get as close as
possible


the record may be .000000700
Kelvin.

*
This is one reason we use the Kelvin scale.
Not only does is the internal energy of a
system proportional to the temperature, you
never have to divide by 0