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Chapter 12
Laws of Thermodynamics
Chapter 12 Objectives
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Internal energy vs heat
Work done on or by a system
Adiabatic process
1st Law of Thermodynamics
2nd Law of Thermodynamics
Isobaric, Isovolumetric, Isothermal
Heat engines
Efficiency of a heat engine
Carnot engine
Entropy
Internal Energy
• Internal energy can be thought of as all the
energy in a system that is not being
transferred as heat.
– This could include nuclear energy, chemical
energy, elastic energy as well as heat that has
not been transferred yet.
• Temperature can often be thought of as a measure
of internal energy.
• This is any amount of energy that cannot be
included as mechanical energy.
– Potential
– Kinetic
Work
W = PV
• Internal energy can be transferred between systems
without transferring heat.
– That would mean that the temperature would not change.
• So the internal energy could be transferred as
mechanical energy in the form of work.
• Recall that work required some displacement to exist,
we also need that fluid to create a displacement.
• So work can only be done when there is a change in
volume.
– The pressure should remain constant.
– If not, then the equation above should be broken down
parts of constant pressure.
Work On or By the System
• Work can be positive or negative, depending
“who” is doing the work.
• The gas does work on the system when the
volume is expanding.
– That means that V is positive, so work is positive.
• When work is being done by the system, the
volume is decreasing.
– So V should be negative, so work will be negative.
Isobaric,
Isovolumetric,
Isothermal
• A system can be isobaric when the pressure is
held constant in that system.
– So cross out P in the equation
• A system can be isovolumetric when the
volume is held constant in that system.
– So cross out V in the equation
• A system can be isothermal when the
temperature is held constant in that system.
– So cross out T in the equation
Adiabatic
• An adiabatic process is one in which no heat
is transferred between the system and the
environment while work is being done.
• Which means the gas has the ability to freely
expand in the container while the container is
completely insulated from its environment.
• Usually involves filling a container with more
gas molecules.
– Such as filling a balloon with air.
1st Law of Thermodynamics
• This is generally known as the Law of
Conservation of Energy.
– So the internal energy of the system cannot be created
or destroyed.
• So the change in internal energy needs to
account for the heat in the gas and whatever
work is done by the gas.
Work done by gas
U = Q - W
Change in
Internal Energy
Heat released or absorbed by gas
Isolated System
• An isolated system is one in which the system
does not interact with its surroundings.
• No interaction means
– No pressure to change the gas pressure
– No volume change of the container
– No temperature due to no transfer of energy.
• No pressure = no work!
• No temperature difference = no heat!
Cyclic Process
• A cyclic process is a process that starts and
finishes at the same state.
– Heat engines are a good example of a cyclic process.
• Air conditioner

• Since the initial and final state of the system is
constant, the internal energy remains the same.
– So Q = W
U = Q - W
Isobaric Process
• An isobaric process remember maintains constant
pressure.
• Since pressure is constant, that allows work to be
done whenever there is a volume change.
• Temperature can also change, so that means heat
can be transferred.
U = Q - W
W = PV
Isovolumetric Process
• An isovolumetric system is one in which the
volume does not change.
• No change in volume means that there is no work
being done.
• So any change in the internal energy is directly
due to the heat being released or absorbed by the
gas.
U = Q - W
W = PV
Isothermal Process
• An isothermal process is one in which the
temperature is kept constant.
– This would mean that the internal energy of the gas must
be kept constant.
• So Uf = Ui
– U = 0
• So any heat released or absorbed by the gas is a
result of work being done.
U = Q - W
Q=W
Adiabatic Process
• Recall that an adiabatic process is one in which there is no
heat transfer and yet there is work being done.
– This process is one in which the number of particles are being
increased.
• Like blowing up a balloon.
• An increase of particles would require the system to do work
to bring those particles in.
– That would use up internal energy to do that work.
• The opposite would be true also when the particles are
released.
– Here the gas would do work on the system by adding gas
molecules to it.
U = Q - W
U = - W
Heat Engine
• A heat engine is any device that converts heat
energy into useful forms of energy.
– Mechanical energy
– Electrical energy
• A heat engine carries some working material (fluid)
that transfers energy from a cold to hot reservoir.
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Steam engine
Internal combustion engine
Refrigerator
Air Conditioner/Furnace
• The net work done by a heat engine is equal to the
difference of the hot and cold reservoirs.
– Hot reservoir can also be thought of as input energy.
– Cold reservoir can also be thought of as output energy.
W = Qh - Qc
2nd Law of Thermodynamics
• It is impossible to construct a heat engine that is
100% efficient.
• Efficiency is found by the ratio of net work done
to the heat absorbed by the hot reservoir (input
energy).
Qh - Qc
W
=
e=
=
Qh
Qh
Qc
1Qh
Carnot Engine
• The Carnot engine is often thought to be the
most efficient heat engine possible.
– Sadi Carnot
• French
– 1796-1832
• The Carnot engine is an engine operating in an
ideal reversible cycle.
– A reversible cycle is one in which at every stage of the
cycle the system is in thermal equilibrium.
• Also called a Carnot cycle.
• Carnot’s Theorem
– No real engine operating between two heat reservoirs
can be more efficient than a Carnot engine, operating
between the same two heat reservoirs.
Efficiency of a
Carnot Engine
• The efficiency of a Carnot engine is based purely
off of the temperature difference between heat
reservoirs of the system.
– Temperature measured in Kelvin.
e=
Th - Tc
Th
Tc
= 1Th
Based on the Carnot cycle, the highest efficiency of
a gasoline internal combustion engine is 30%!
Entropy and Disorder
• Entropy is a measure of the disorder found in a
thermodynamic system.
– Larger the entropy, the more disorder of the molecules
and their behavior.
• Based on probability, systems with high disorder
are much more likely to happen in nature.
– With that said, the entropy of the Universe is always
increasing.