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Chapter 11
Laws of Thermodynamics
Chapter 11 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
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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.
W = PV
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
U = Q - W
• 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
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.
W = PV
U = Q - W
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 wasted 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
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.