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Examples of power

Examples of power are limited only by the imagination, because there are as many types as there are forms of work and energy. (See [link] for some examples.) Sunlight reaching Earth’s surface carries a maximum power of about 1.3 kilowatts per square meter ( kW/m 2 ) . size 12{ \( "kW/m" rSup { size 8{2} } \) "." } {} A tiny fraction of this is retained by Earth over the long term. Our consumption rate of fossil fuels is far greater than the rate at which they are stored, so it is inevitable that they will be depleted. Power implies that energy is transferred, perhaps changing form. It is never possible to change one form completely into another without losing some of it as thermal energy. For example, a 60-W incandescent bulb converts only 5 W of electrical power to light, with 55 W dissipating into thermal energy. Furthermore, the typical electric power plant converts only 35 to 40% of its fuel into electricity. The remainder becomes a huge amount of thermal energy that must be dispersed as heat transfer, as rapidly as it is created. A coal-fired power plant may produce 1000 megawatts; 1 megawatt (MW) is 10 6 W size 12{"10" rSup { size 8{6} } " W"} {} of electric power. But the power plant consumes chemical energy at a rate of about 2500 MW, creating heat transfer to the surroundings at a rate of 1500 MW. (See [link] .)

A distant view of a coal-fired power plant with clearly visible cooling towers generating electric power and emitting a large amount of gases.
Tremendous amounts of electric power are generated by coal-fired power plants such as this one in China, but an even larger amount of power goes into heat transfer to the surroundings. The large cooling towers here are needed to transfer heat as rapidly as it is produced. The transfer of heat is not unique to coal plants but is an unavoidable consequence of generating electric power from any fuel—nuclear, coal, oil, natural gas, or the like. (credit: Kleinolive, Wikimedia Commons)
Power output or consumption
Object or Phenomenon Power in Watts
Supernova (at peak) 5 × 10 37 size 12{5 times "10" rSup { size 8{"37"} } } {}
Milky Way galaxy 10 37 size 12{"10" rSup { size 8{"37"} } } {}
Crab Nebula pulsar 10 28 size 12{"10" rSup { size 8{"28"} } } {}
The Sun 4 × 10 26 size 12{4 times "10" rSup { size 8{"26"} } } {}
Volcanic eruption (maximum) 4 × 10 15 size 12{4 times "10" rSup { size 8{"15"} } } {}
Lightning bolt 2 × 10 12 size 12{2 times "10" rSup { size 8{"12"} } } {}
Nuclear power plant (total electric and heat transfer) 3 × 10 9 size 12{3 times "10" rSup { size 8{9} } } {}
Aircraft carrier (total useful and heat transfer) 10 8 size 12{"10" rSup { size 8{8} } } {}
Dragster (total useful and heat transfer) 2 × 10 6 size 12{2 times "10" rSup { size 8{6} } } {}
Car (total useful and heat transfer) 8 × 10 4 size 12{8 times "10" rSup { size 8{4} } } {}
Football player (total useful and heat transfer) 5 × 10 3 size 12{5 times "10" rSup { size 8{3} } } {}
Clothes dryer 4 × 10 3 size 12{4 times "10" rSup { size 8{3} } } {}
Person at rest (all heat transfer) 100 size 12{"100"} {}
Typical incandescent light bulb (total useful and heat transfer) 60 size 12{"60"} {}
Heart, person at rest (total useful and heat transfer) 8 size 12{8} {}
Electric clock 3 size 12{3} {}
Pocket calculator 10 3 size 12{"10" rSup { size 8{-3} } } {}

Power and energy consumption

We usually have to pay for the energy we use. It is interesting and easy to estimate the cost of energy for an electrical appliance if its power consumption rate and time used are known. The higher the power consumption rate and the longer the appliance is used, the greater the cost of that appliance. The power consumption rate is P = W / t = E / t size 12{P= {W} slash {t} = {E} slash {t} } {} , where E size 12{E} {} is the energy supplied by the electricity company. So the energy consumed over a time t size 12{t} {} is

E = Pt. size 12{E= ital "Pt"} {}

Electricity bills state the energy used in units of kilowatt-hours ( kW h ) , size 12{ \( "kW" cdot h \) ,} {} which is the product of power in kilowatts and time in hours. This unit is convenient because electrical power consumption at the kilowatt level for hours at a time is typical.

Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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