Thermal

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Thermal Kinetic Lecture 16 Isothermal and
Adiabatic processes
LECTURE 16 OVERVIEW
Isothermal and Adiabatic work
Is work a function of state?

What distinguishes heat from work?
Using thermal processes to do work Intro. to the
Carnot cycle
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Last time.
Heat, work, and the 1st law.
Adiabatic processes and adiabatic work.
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Work done by expanding gases path dependence
T, V, and P are functions of state. Is W also a
function of state?
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Heat, work and the 1st law Adiabatic
compression and expansion of an ideal gas
However, weve shown in Section 2 that CP CV
R (Eqn. 2.45). Hence, we can write the equation
above as
Equation of an adiabatic
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PV curve for an adiabatic and an isothermal
process
P
Adiabatic
At each point (P, V), the adiabatic for an ideal
gas has a slope g times that of an isotherm for
an ideal gas.
Isotherm
V
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PV curve for an adiabatic process
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PV curve for an adiabatic process
but PV nRT for an ideal gas

The adiabatic for an ideal gas has a slope g
times that of an isotherm.
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Adiabatic work

• At the beginning of the 19th century it was
  assumed that heat was a substance called caloric
  which flowed between bodies.
• Prompted by measurements carried out by Benjamin
  Thompson, Joule wanted to determine the precise
  form of heat.

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Adiabatic work
No matter how the adiabatic work was performed,
it always took the same amount of work to take
the water between the same two equilibrium states
(whose temperatures differed by DT)
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Adiabatic work
(c is a constant)
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Adiabatic work
Therefore, we can rewrite the expression for W as
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The distinction between heat and work
If both heat and work increase the internal
energy of a system, what is the distinction
between the two at the microscopic level?
Heat changes the populations of the energy levels
(so have change in entropy because theres a
change in the number of accessible microstates.)
Works changes the energies, with the populations
staying the same.
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Using thermal processes to do work heat engines

Carnot noted that work is obtained from an engine
because there are heat sources at different
temperatures. Furthermore, he realised that heat
could also flow from a hot to a cold body with no
work being done.
A temperature difference may be used to produce
work OR it can be squandered as heat.
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The most efficient process the Carnot cycle
In an ideal engine the temperature difference
between the two reservoirs should yield the
maximum amount of work possible. Carnot
realised that this meant that all transfers of
heat should be between bodies of nearly equal
temperature. The Carnot engine involves
reversible processes (these are the most
efficient processes in terms of exploiting a
temperature difference to do work).
TH
QH
W
QL
TL
Heat supplied from high temp. reservoir QH Heat
rejected into lower temp. reservoir QL
A Carnot engine operates between only two
reservoirs and is reversible. All the heat that
is absorbed is absorbed at a constant high
temperature (QH) and all the heat that is
rejected is rejected at a constant lower temp.
(QL).
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The most efficient process the Carnot cycle
Carnot engine is an idealisation. Well use an
ideal gas as our working substance. Carnot cycle
may be constructed from a combination of
adiabatic and isothermal compressions and
expansions.
P
A
Adiabatic
Isotherm
Animation
V