2 
1
Part II
First Law of
Thermodynamics
Introduction
The
first law
deals with macroscopic properties,
work
,
energy
,
enthalpy
, etc.
One of the most fundamental laws of nature is the
conservation of energy
principle
. It simply states that
•
during an interaction, energy can change from one form to
another but the total amount of energy remains constant. That
is, energy cannot be created or destroyed. Or,
•
during an interaction between a system and its surroundings,
the amount of energy gained by the system must be exactly
equal to the amount of energy lost by the surroundings. A rock
falling off a cliff, for example, picks up speed as a result of its
potential energy being converted to kinetic energy.
The first law of thermodynamics is simply an expression of the
conservation of energy principle, and it asserts that
energy
is a
thermodynamic property.
Energy can cross the boundary of a closed system in two distinct forms:
heat
and
work
. It is important to distinguish between these two forms of energy.
Therefore, they will be discussed first, to form a sound basis for the
development of the first law of thermodynamics.
We can use the principle of conservation of energy to define a function
U
called the
internal energy.
When a closed system undergoes a process by which
it passes from state A to state B, if the only interaction with its surroundings is
in the form of transfer of heat
Q
to the system, or performance of work
W on
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the system, the change in
U
will be
∆
U = U
B
– U
A
= Q + W
21
Note:
•
In Equation 21 we have defined
W
as
the work done on the system and
Q
is added
to the system. If we had defined
W
as
work done by the system, Equation 21
would become
U
=
Q W.
•
For an isolated system there is no heat or work transferred with the surroundings,
thus, by definition
W
=
Q
= 0 and therefore
U
= 0.
•
The first law of thermodynamics states that this energy difference
U
depends
only on the initial and final states, and not on the path followed between them.
Both
Q
and
W
have many possible values, depending on exactly how the system
passes from A to B, but
Q
+
W
=
U
is invariable and independent of the path. If
this were not true, it would be possible, by passing from A to B along one path and
then returning from B to A along another, to obtain a net change in the energy of
the closed system in contradiction to the principle of conservation of energy.
•
For a differential change, Equation 21 becomes
dU = dQ +dW
22
•
For a cyclic process, A
→
B
→
A, when the system returns to state A, it has the same
U
, thus
∫
=
0
dU
23
Next we will take a look separately at the heat transferred (
dQ
) and the
work (
dW
) exchanged between the system and the surroundings.
21 Heat Transfer
Heat is defined as
the form of energy that
is
transferred between two systems
(or a system and its surroundings) by virtue of a temperature difference.
That
is, an energy interaction is heat only if it takes place because of a temperature
difference. Then it follows that there cannot be any heat transfer between two
systems that are at the same temperature
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 Spring '10
 PhD
 Thermodynamics, Energy, Work, Heat, Heat Transfer

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