72 Calories (Units of Energy)
Joule turned his attention to the conversion of mechanical energy into heat. In real life
a moving body (with the mechanical energy of momentum) eventually stopped. What happened to its energy? He designed a series of experiments using water to measure the conversion of mechanical motion into heat.
Two of Joule’s experiments became famous. First, he submerged an air-filled copper
cylinder in a tub of water and measured the water temperature. He then pumped air into the
cylinder until it reached 22 atmospheres of pressure. The gas law said that the mechanical
work to create this increased air pressure should create heat. But would it? Joule measured a
0.285°F rise in water temperature. Yes, mechanical energy had been converted to heat.
Next, Joule attached paddles onto a vertical shaft that he lowered into a tub of water.
Falling weights (like on a grandfather clock) spun the paddles through the tub’s water. This
mechanical effort should be partially converted to heat. But was it?
His results were inconclusive until Joule switched from water to liquid mercury. With
this denser fluid, he easily proved that the mechanical effort was converted to heat at a fixed
rate. Liquid was heated by merely stirring it.
Joule realized that all forms of energy could be converted into equivalent amounts of
heat. He published these results in 1843 and introduced standard heat energy units to use for
calculating these equivalences. Since then, physicists and chemists typically use these units
and have named them joules. Biologists prefer to use an alternate unit called the calorie
(4.18 joules = 1 calorie). With this discovery that any form of energy could be converted
into an equivalent amount of heat energy, Joule provided a way to advance the study of
energy, mechanics, and technologies.
Fun Facts: The calories on a food package are actually kilocalories, or
units of 1,000 calories. A kilocalorie is 1,000 times larger than the calorie
used in chemistry and physics. A calorie is the amount of energy needed
to raise the temperature of 1 gram of water 1 degree Celsius. If you burn
up 3,500 calories during exercise, you will have burned up and lost one
pound. However, even vigorous exercise rarely burns more than 1,000
calories per hour.
More to Explore
Cardwell, Donald, ed. The Development of Science and Technology in Nineteenth-Century Britain. London: Ashgate Publishing, 2003.
———. James Joule: A Biography. New York: St. Martin’s Press, 1997.
Joule, James. The Scientific Papers of James Prescott Joule. Washington, DC: Scholarly Press, 1996.
Smith, Crosbie. Science of Energy. Chicago: University of Chicago Press, 1999.
Steffens, Henry. James Prescott Joule and the Concept of Energy. Sagamore Beach,
MA: Watson Publishing, 1999.