The internal energy of a substance can be changed in different ways. Work can transfer energy to a substance and increase its internal energy.

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The internal energy of a substance can be changed
in different ways. Work can transfer energy to a
substance and increase its internal energy.
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Heat can be lost by the substance, which results
in a decrease of internal energy.
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Energy can also be transferred to the substance
as heat and from the substance as work.
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Energy is added to substances or groups of
substances. It is also removed from these
substances. Such a substance or combination of
substances is called a system.
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The surroundings with which the system interacts
is called the environment.
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Work done on or by a gas is the pressure
multiplied by the change in volume. W P?V
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An engine cylinder has a cross-sectional area of
0.010 m2. How much work can be done if a gas
exerts a constant pressure of 7.5 x 105 Pa and
moves the piston 0.040 m?
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We have been discussing three quantities and how
they relate to each other internal energy (U),
heat (Q), and work (W). The study of how these
relate is called thermodynamics.
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We can simplify our discussion by considering
situations where one of these properties (U, Q,
or W) does not change.
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No work is done in a constant volume process.
W P?VSuch processes are isovolumetric.
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From PV/T, a change in P without a change in V
requires a change in T.
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When the temp of a gas changes without a change
in volume, no work is done on or by the system.
(Energy must be added to or taken from the
system by some other means.)
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Internal energy is constant in a constant
temperature process (isothermal). This remains
constant even as energy is transferred to or from
the system as heat or work.
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When heat energy is not transferred in a process
it is called adiabatic. Any change in internal
energy is due to work done on or by the system.
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The first law of thermodynamics states that any
change in internal energy is equal to the energy
transferred to or from the system as heat and the
energy transferred to or from the system as
work. ?U Q W
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This is another statement of the law of
conservation of energy. In this case, it is not
just mechanical energy that is conserved, but all
the energy of a system. ?U Q W
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Q is positive if heat is added to a system.Q is
negative is heat is removed from a system.
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?U Q WW is positive if work is done on a
system (gas compression).W is negative if work
is done by a system (gas expansion).
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Important Note The signs for work done on the
system () and work done by the system (-) are
the opposite listed in your book to reflect
changes in the AP test.
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A gas in a cylinder with a movable piston is
submerged in ice water. The initial temperature
of the gas is 0C. 1200 J of work is done by a
force that slowly pushes the piston inward. A)
Is this process isothermal, adiabatic, or
isovolumetric?B) How much energy is transferred
as heat?
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A total of 135 J of work is done on a gas through
compression. If the internal energy of the gas
increases by 114 J, what is the total amount of
energy transferred as heat? Has energy been added
to or removed from the gas as heat?
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A refrigerator does work to create a difference
in temperature between its closed interior and
its environment. This is a cyclic process and the
change in internal energy of a system is zero in
a cyclic process.
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In a cyclic process?Unet 0 and Qnet
WnetThe difference in the transfer of heat from
the system ,Qh, and the transfer of heat to the
system ,Qc, is equal to Wnet Qh - Qc Wnet.
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A refrigerator uses work to remove heat from the
system. A heat engine does the opposite, it
uses heat to do mechanical work.
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It is still a cyclic process, and the formulas
and relationships are still the same ?Unet 0
and Qnet Wnet Qh - Qc Wnet
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The second law of thermodynamics states that no
cyclic process that converts heat entirely into
work is possible.
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W can never be equal to Qh, there must always be
a value Qc gt 0 indicating a loss of heat to the
environment.
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The efficiency of a thermodynamic system is
calculated using these equations eff Wnet/Qh
eff (Qh - Qc)/Qh eff 1 - Qc/Qh
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A heat engine can only be 100 efficient if no
energy is removed as heat Qc 0.Engines are
most efficient if Qh is high and/or Qc is low.
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Find the efficiency of a gasoline engine that,
during one cycle, receives 204 J of energy from
combustion and loses 153 J as heat to the exhaust.
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In thermodynamics, a system tends to change from
a very ordered set of energies to one where there
is less order. The measure of a systems
disorder is called entropy.
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Systems with maximum disorder are
favored.Greater disorder means there is less
energy available to do work.
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The second law of thermodynamics, (No cyclic
process that converts heat entirely into work is
possible.), can be stated in terms of entropy
The entropy of the universe increases in all
natural processes.
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Entropy can increase or decrease within a system.
Entropy can decrease for parts of a system as
long as this decrease is offset by a greater
increase in entropy elsewhere in the universe.
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