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8.5 Power 257

The above equations all refer to mechanical power. In general, power is the rate at
which energy is transferred from one form of energy to another or the rate at which energy
is transported from one place to another. For instance, an automobile engine converts
chemical energy of fuel into mechanical energy and thermal energy. A nuclear power
plant converts nuclear energy into electric energy and thermal energy. And a high-
voltage power line transports electric energy from one place to another.Table 8.2 gives
some examples of different kinds of power.

TABLE 8.2 SOME POWERS

26
Light and heat emitted by the Sun 3.9 10 W
13
Mechanical power generated by hurricane (a) 2 10 W
12
Total power used in United States (average) 2 10 W
9
Large electric power plant 10 W
8
Jet airliner engines (Boeing 747) (b) 2.1 10 W
5
Automobile engine 1.5 10 W
3
Solar light and heat per square meter at Earth 1.4 10 W
3
Electricity used by toaster 1 10 W
2
Work output of man (athlete at maximum) 2 10 W
2
Electricity used by light bulb 1 10 W
Basal metabolic rate for man (average) 88 W
Heat and work output of bumblebee (in flight) (c) 2 10 2 W
Atom radiating light 10 10 W

(a) (b) (c)

In the previous example, part of the horse’s work was converted into heat by the
friction between the sled and the snow, and part was converted into gravitational poten-
tial energy. In the following example, gravitational potential energy is converted into
electric energy.

Each of the four generators (Fig. 8.17) of the Brown Mountain
EXAMPLE 10 Concepts
hydroelectric plant generates 260 MW of electric power. in
Context
When generating this power, at what rate does the power plant take water from the
upper reservoir? How long does a full reservoir last? See the data in Example 5.
SOLUTION: We will assume that all of the potential energy of the water in the
upper reservoir, at a height of 320 m, is converted into electric energy.The electric