Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

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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