13.3 Understanding What We Mean by Power 381
of the system. As you learn more about energy you will also learn that according to the second
law of thermodynamics, unfortunately you cannot even break even, because there are always
losses associated with processes. We will discuss the effect of losses in terms of performance and
efficiency of various systems later in this chapter.Example 13.5 Determine the change in the total energy of the system shown in Figure 13.9. The heater puts
150 W ( J/s) into the water pot. The heat loss from the water pot to the atmosphere is 60 W.
Calculate the change in total energy of water in the pot after 5 minutes.60 watts
Water
150 watts
■Figure 13.9 A schematic diagram for Example 13.9.
We can use Equation (13.8) to solve this problem.
Note, for this problem, there is no change in the kinetic energy and potential energy of the sys-
tem (water inside the pot). Consequently, the change in total energy is equal to the change in
internal energy of water. Moreover, the rise in the internal energy manifests itself in the rise of
the water temperature.13.3 Understanding What We Mean by Power
In Section 13.1, we reviewed the concept of work as presented in Chapter 10 and explained the
different forms of energy. We now consider what is meant by the termpower. Poweris formally
defined as the time rate of doing work, or stated simply, the required work, or energy,
divided by the time required to perform the task (work).¢E27 kJ
1 150 J/s 21 300 s 2 1 60 J/s 21 300 s 2 ¢E
W 0
QW¢E
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