Measuring Energy and Power

Energy and power are measured in a variety of units. Units of energy used in the United States include the foot­pound, the Btu, and the kilowatt-hour. The foot-pound is the energy needed to lift a 1-pound weight a distance of 1 foot. The Btu, or British thermal unit, is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. The kilowatt – hour is the amount of energy needed to supply one kilowatt of power for one hour (1 kilowatt equals 1,000 watts).

The international unit of energy is the joule-the amount of energy used, or work done, when a force of one newton acts through a distance of one meter. Another way to describe a joule is to say it is the energy needed to lift a small apple 3.28 feet (1 meter) off the ground.

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JOHN PRESCOTT JOULE (1818-1889)

The joule, the international unit for measuring energy, is named after English physicist John Prescott Joule. Joule studied heat and mechanical work (the work done in moving objects) and how they are connected.

His research led to the law of conser­vation of energy. Joule also found the link between the electric current flowing through something and the amount of heat given out. This con­nection is known as Joule’s Law, or the Joule Effect.

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Power is the amount of work done per second. It is a measurement of how fast energy changes from one form to another. The international unit of power is the watt. One watt is the same as one joule per second. A 100-watt light bulb changes 100 joules of electrical energy into light and heat every second. A large airliner’s engine is as powerful as about 2 million of these light bulbs, or about

200,0 kilowatts.

Power also can be measured in horsepower. One horsepower used to be
the power of an actual horse. Today, it is the same as about 746 watts or 550 foot-pounds per second. A big airliner engine produces approximately 270,000 horsepower—more power than one – quarter of a million horses!

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SEE ALSO:

• Bird • Force • Fuel • Gravity • Jet

and Jet Power • Laws of Motion

• Weight and Mass

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