380 Chapter 13 Energy and Power
Qnet heat transfer into the system ( Qin Qout) in joules ( J)
Wnet work done by the system ( Wout Win) in joules ( J)
¢Enet change in total energy of the system in joules ( J), whereErepresents the sum
of the internal energy, kinetic energy, potential energy, elastic energy, and other
forms of energy of the system.
There is a sign convention associated with Equation (13.8) that must be followed carefully. Heat
transfer into the system, or the work done by the system, is considered a positive quantity,
whereas the heat transfer out of the system, or the work done on the system, is a negative quan-
tity. The reason for this type of sign convention is to show that work done on the system increases
the total energy of the system, while work done by the system decreases the total energy of the
system. Also note that using this sign convention, heat transfer into the system increases the
total energy of the system, while heat transfer out of the system decreases its total energy.
You may also think of the first law of thermodynamics in the following manner: When it
comes to energy, the best you can do is to break even. You cannot get more energy out of a sys-
tem than the amount you put into it. For example, if you put 100 J into a system as work, you
can get 100 J out of the system in the form of change in internal, kinetic, or potential energy
gg
gg
E
Qin
Wdone
■Figure 13.8
The first law of thermodynamics
for a system having a fixed mass.
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