THERMODYNAMICS
Thermodynamics is the study of heat flow.
Thermodynamics explains how heat is transformed to mechanical energy
Thermodynamics explains how energy is conserved in a system.
Thermodynamics explains energy efficiency.
Thermodynamics explains why energy output can’t exceed energy input
(perpetual motion)
I. WHAT IS TEMPERATURE?
TEMPERATURE corresponds to the average kinetic energy (KE) of a substance.
It is wrong to say temperature is a measure of heat energy though a thermometer
usually tracks with the input or output of heat. The input or output of heat causes
the KE of a substance to change.
ABSOLUTE TEMPERATURE
•  The absolute temperature scale is called the KELVIN SCALE.
•  At 0 K, all thermal motion (KE) ceases. This is the lower limit of temperature.
•  At this temperature, if it were possible to achieve it, no addition energy can be
extracted from a substance and hence no lower temperature Is to be expected.
• This is -273.15 oC on the Celcius scale.
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Statement of the 3 laws
1ST LAW (conservation of energy):
The
change
in
a
system's
internal
energy
is
equal
to
the
difference
between
heat
added
to
the
system
from
its
surroundings
and
work
done
by
the
system
on
its
surroundings.
2nd LAW:
It is impossible for a process to have as its sole result the
transfer of heat from a cooler body to a hotter one.
3rd LAW:
It is impossible to reduce any system to absolute zero in a finite
series of operations.
ΔU = U f − U i = Q − W
U is the internal energy of
a system. Q is heat added
from surroundings. W is work by
the system. This law basically means that
if you add Q to a system either U will change
or the system will do work.
ΔS ≥ 0
S is entropy. If a cold body
gave up heat to a hot body, the entropy
of the cold body would decrease. Not
possible, not spontaneous. Could be done ONLY
by doing work on the system (like a refrigerator)
S=0
This basically means you
cannot remove all of the thermal
energy from a substance. If you could,
the entropy, S, of a system would
equal zero at absolute zero
They seem hard to understand at first but they actually make sense
with your everyday experience
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II. Explanation of the laws
A. 1st Law
ΔU = U f − U i = Q − W
The 1st law just says that if heat, Q, is added to a system, the internal energy of the
SYSTEM will change and/or the system will do work, W, on the surroundings
INTERNAL ENERGY is the energy associated with random motion of the particles
(atoms, molecules) in a substance. It is ENERGY at the MICROSCOPIC LEVEL.
A SYSTEM is a collection of atoms, molecules, a substance, any object, an engine, etc
Whatever “thhing” we are interested in determining how it interacts with heat.
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1st law, continued
The lst law is a way to state the CONSERVATION OF ENERGY, that energy
can be neither created or destroyed. It can change forms.
ΔU = U f − U i = Q − W
Suppose we design an experiment where the heat (Q) is PREVENTED from
entering or leaving the system. This system is said to be ADIABATIC.
On a practical level, you can think of an adiabatic system as one that is wrapped
in insulation. Your house might be an example, though it is far from perfectly
adiabatic.
The 1st law becomes:
ΔU = −W
when Q = 0
This means that the internal energy of a system is influenced only by the amount
of work it does to the surroundings or the amount of work the surroundings does on it.
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1st law, continued
no heat flow
no heat flow
Surrounding does the
work. Internal energy
INCREASES
System does the
work. Internal energy
DECREASES
If we rapidly compress a gas in a cylinder (left picture), and not let any heat
leave the system, the internal energy increases as the compressed gas warms
If we allow the gas to rapidly expand (right picture), and again not allow heat to
transfer, the internal energy of the system decreases as the gas cools.
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B. 2nd law
The second law can be stated in many different ways. It is often badly misinterpreted
because some of the concepts used to describe it are obscure to most people.
The second law says that heat cannot spontaneously flow from a cold to a hot body.
This seems obvious. But there are an awful lot of physical and chemical implications
in that statement.
A system free of external influences BECOMES MORE RANDOM OVER TIME.
This is a very general statement of the 2nd law and is the one that is
sometimes misunderstood.
This statement of the law introduces to a new concept called ENTROPY.
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2nd law continued
Let’s re-write the previous statements of the 2nd law and add one more. I think the
one in the middle “bridges” the other two.
Heat cannot spontaneously flow from a cold to a hot body.
You cannot create a heat engine which extracts heat and
converts it all to useful work.
A system free of external influences BECOMES MORE RANDOM
OVER TIME.
In other words, a system is never completely EFFICIENT at converting input energy
to output energy.
There is unavoidable LOSS of heat energy (loss means not used to do useful work)
associated with ENTROPY.
YOU CAN”T BUILD A PERPETUAL MOTION MACHINE !
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2nd law, continued
Heat engines and efficiency
A heat engine is any device that converts internal energy into mechanical work.
The idea is to TRANSFER heat from a HOT body to a COLD body and EXTRACT
WORK along the way.
Unfortunately, not all of the heat gets converted to USEFUL WORK (entropy again).
The diagram to the left shows the principle behind
a heat engine. Heat is transferred to the cold
sink. Work is extracted.
T − TL Ti − T f
e= H
=
TH
Ti
The Carnot efficiency is:
Work done
e=
Q added
e=
(Qi − Q f )
Qi
this equation tells you how efficient your
heat engine can be.
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PV Work
The product of a system’s pressure
and change in volume during
an expansion or compression of
gas in the system is known as
PV work.
Work = PΔV = P(Vf – Vi)
In the Carnot cycle, the highest
PV work occurs along path
AB and CD, where added or lost
heat causes the gas volume to
expand or contract isothermally.
The units of PV work could be something
like liter-atm, which are legitimate, if
weird, units of work energy (like Joules).
1 l-atm = 101.325 J
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2nd law, continued
On the other hand, if you want to remove
heat FROM a cold sink, your system MUST
have WORK done on it to “pump” the
heat away from the cold sink toward the
hot sink.
Refrigerators and heat pumps work this
way.
ENTROPY
Entropy is mathematically defined as:
Q
J
ΔS =
units :
T
K
Entropy is often described as a measure DISORDER or RANDOMNESS.
Does that make sense when the units are heat energy per kelvin? Entropy is
related to heat energy yet we describe it as “disorder”.
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2nd law and entropy, continued
Here’s a way to think about entropy:
When a system does work, some of the energy required DOES NOT convert to
useful work
The non-useful energy (waste, inefficiency of the system) goes toward heating the
system or surroundings, just causing more wiggling and jostling of the particles
in and around the system.
Energy, freely distributed as such, creates disorder. This disorder AND ITS RELATIONSHIP
TO HEAT is defined as ENTROPY.
uses energy
requires work input
Systems with work input can maintain order and decrease entropy. But the
decrease in entropy for one system increases it for another system.
THE ENTROPY OF THE UNIVERSE TENDS TO INCREASE OVER TIME.
The CHANGE in entropy for a system can be + or – but the entropy
of a system at any given time is always + (in the direction of “disorder”)
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3rd law
The 3rd law says that a perfectly pure substance, in a perfect crystalline structure,
will have an entropy value = 0 at ABSOLUTE ZERO.
In other words, remove all the energy of the system, and the system has no
opportunity for a disordered state.
As a substance approaches absolute zero, its entropy decreases. According to the
2nd law, however, the entropy of the surroundings will increase.
As a practical matter, the 3rd law sets a lowest possible energy (or temperature) level
that a system can attain.
It also says that at any temperature above absolute zero, the entropy of a system will
be greater than 0 (that is, +).
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