Deakin University 2003
Topic 2
Energy conservation: The
First Law of Thermodynamics
Throughout this topic you will need to refer to sections 1.6–1.8 in your
textbook.
Conservation of energy
The First Law of Thermodynamics simply introduces the concept of
‘conservation of energy’. In other words,
the (amount of) energy cannot be
created or destroyed
, that is, energy never dies, it just changes form. This law
cannot be proved mathematically. It is based on experimental observations and
nothing has yet been found that violates this law. A nuclear reaction in which
mass is converted into energy is not considered a violation of the law; rather
mass is treated as yet another form of energy.
In its basic form, the First Law of Thermodynamics can be stated as follows:
When a system undergoes a thermodynamic cycle then the
net
heat supplied to
the system from its surroundings is equal to the
net
work done by the system
on its surroundings, or, mathematically,
Σ
dQ =
Σ
dW
(2.1)
where
Σ
represents the sum for a complete cycle.
Equation 2.1 is NOT meant to imply that if a certain amount of work is done
on a system, then it is all converted into heat or, conversely, if a certain
amount of heat is supplied to a system, then it is all converted into work.
Equation 2.1 simply means that if some work is converted into heat or some
heat is converted into work then the relationship between the heat and work so
converted will be of the form W = Q.
Actually, it is possible to convert work completely into heat by friction, for
example. The reverse process of converting heat completely into work
(through a cycle) is impossible and the reason will be explained in topic 5.
Now please read through section 1.6 and example 1.3 in your textbook.
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Principles of Thermodynamics
Deakin University 2003
The nonflow energy equation
In a nonflow process, an extension of equation 2.1 may be used:
W + Q =
Δ
U
(2.2)
This equation is known as the
closed system energy equation
.
This equation takes into account that not all the energy needs to be of a heat–
work or work–heat conversion form. Some energy may be concerned with an
internal energy change,
Δ
U, as equation 2.2 shows. Now read section 1.7 in
your textbook and make sure you understand the two examples given
(examples 1.4 and 1.5). Here, I will expand the discussion of the First Law of
Thermodynamics and give you some more solved examples.
Note:
Equation 2.1 is talking about the net heat and work involved in a
complete cycle, that is, the system goes back to its original state, while
equation 2.2 gives the relationship between heat work and internal energy
change in a nonflow process (reversible or irreversible).
To facilitate understanding of the first law, consider the following analogy.
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 Spring '09
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 Dynamics, Thermodynamics, Energy, Deakin University

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