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Thermal Physics
Topic 10.2 Thermodynamic
Systems and Concepts
Thermodynamics
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Is the name given to the study of
processes in which thermal energy is
transferred as heat and as work.
It had its foundations with engineers in
the 19th century who wanted to know
what were the limitations of the Laws of
physics with regard to the operation of
steam engines and other machines that
generate mechanical energy.
Thermodynamics

Treats thermal energy from the
macroscopic point of view in that it
deals with the thermodynamic variables
• pressure, volume, temperature

And change in internal energy in
determining the state of a system.
Heat

Can be transferred between a system
and its environment because of a
temperature difference.

Another way of transferring energy
between a system and its environment
is to do work on the system or allow
work to be done by the system on the
surroundings.
Definitions
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In order to distinguish between heat and
work in thermodynamic processes
Heat is defined as a process in which
thermal energy is transferred due to a
temperature difference.
Work is defined as the process in which
thermal energy is transferred by means
that are independent of a temperature
difference.
System

In thermodynamics the word system is used
often.

A system is any object or sets of objects that
is/are being investigated.

The surroundings will then be everything in
the Universe apart from the system.
For example
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When a volume of gas in a cylinder is
compressed with a piston
Then the system is the cylinder-gaspiston apparatus
And the surroundings is everything else
in the Universe.
A closed system
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Is one in which no mass enters or
leaves the system.
It is an isolated system if no energy of
any kind enters or leaves the system.
Most systems are open systems
because of the natural dynamic
processes that occur in the Universe.
Work Done by a Gas
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Consider a mass of gas with pressure p
enclosed in a cylinder by a piston of
cross-sectional area A
The pressure, p, on the piston = force
per unit area
Therefore, the force on the piston, F, is
given by
F =pA
So that

Suppose the piston is moved a distance
l when the gas expands.

Normally, if the gas expands, the
volume increases and the pressure
decreases, as was determined from
Boyle's Law for ideal gas.
Expansion of a Gas at
Constant Pressure
Surface area, A
Pressure, p
cylinder
l
However

If the distance l is small l, then the
pressure can be considered constant.

If the pressure is constant then the force
F will be constant.
And
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The work done by the gas is
W = Fl
but since pressure F = pA
W = pA l
but since volume V = A l
 W = pV
That is

(work done / J) =
 (pressure / Nm-2) x (volume change / m3)
 So that,
 W = p V = p(V2-Vl)
 The sign of the work done by the gas
depends on whether volume change is
positive or negative.
Explanation

When a gas expands, then work is done
by the gas, and the volume increases.
As V is positive, then W is positive.
More

This equation is also valid if the gas is
compressed.
 In the compression, work is done on the gas
and the volume is decreased.
 Therefore, V is negative which means that
W will be negative.
 From the first law of thermodynamics this
means that positive work is done on the gas.
Internal Energy

Internal energy U was defined as the sum
total of the potential energy and kinetic
energy of the particles making up the system.

From a microscopic viewpoint, the internal
energy of an ideal gas is due to the kinetic
energy of the thermal motion of its molecules.
Internal Energy of an Ideal Gas
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There are no intermolecular forces and
thus there cannot be any increase in
potential energy.

Therefore a change in the temperature
of the gas will change the internal
energy of the gas.
More

From the macroscopic point of view of
thermodynamics, one would expect that
the internal energy of the system would
be changed if:
•
•
•
•
work is done on the system
work is done by the system
thermal energy is added to the system
thermal energy is removed from the
system
Internal Energy


is a property of the system that depends
on the "state" of the system.
(In thermodynamics, a system is said to
have changed "state" if some
macroscopic property of the system has
changed e.g. phase, temperature,
pressure, volume, mass, internal
energy).
What can change a system
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Heat and work can change the state of
the system but they are not a property
of the system.
They are not characteristic of the state
itself but rather they are involved in the
thermodynamic process that can
change the system from one state to
another.
The Absolute Value
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The absolute value for internal energy is
not known.
This does not cause a problem as one
is mainly concerned with changes in
internal energy
Denoted by U, in thermodynamic
processes