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Cal o ries (Units of En ergy)
Cal o ries (Units of En ergy)
CALORIES (Units of En ergy)
Year of Dis cov ery: 1843
What Is It? All forms of en ergy and me chan i cal work are equiv a lent and can
be con verted from one form to an other.
Who Dis cov ered It? James Joule
Why Is This One of the 100 Great est?
Why Is This One of the 100 Great est?
We now know that me chan i cal work, elec tric ity, mo men tum, heat, mag netic force,
etc., can be con verted from one to an other. There is al ways a loss in the pro cess, but it can be
done. That knowl edge has been a tre men dous help for the de vel op ment of our in dus tries
and tech nol o gies. Only 200 years ago, the thought had not occurred to anyone.
James Joule dis cov ered that ev ery form of en ergy could be con verted into an equiv a -
lent amount of heat. In so do ing, he was the first sci en tist to come to grips with the gen eral
con cept of en ergy and of how dif fer ent forms of en ergy are equiv a lent to each other. Joule’s
dis cov ery was an es sen tial foun da tion for the dis cov ery (40 years later) of the law of con ser -
va tion of en ergy and for the de vel op ment of the field of thermodynamics.
How Was It Dis cov ered?
How Was It Dis cov ered?
Born on Christ mas eve, 1818, James Joule grew up in a wealthy brew ing fam ily in
Lancashire, Eng land. He stud ied sci ence with pri vate tu tors and, at the age of 20, started to
work in the fam ily brew ery.
Joule’s first self-ap pointed job was to see if he could con vert the brew ery from steam
power to new, “mod ern” elec tric power. He stud ied en gines and en ergy sup plies. He stud -
ied elec tri cal en ergy cir cuits and was fas ci nated to find that the elec tri cal wires grew hot
when cur rent ran through them. He re al ized that some of the elec tri cal en ergy was be ing
converted into heat.
He felt it was im por tant for him to quan tify that elec tri cal en ergy loss and be gan ex per -
i ments on how en ergy was con verted from elec tric ity to heat. Of ten he ex per i mented with
lit tle re gard for safety—his or oth ers. More than once, a ser vant girl col lapsed un con scious
from elec tri cal shocks dur ing these ex per i ments. While he never con verted the brew ery to
elec tri cal power, these ex per i ments turned his fo cus to the pro cess of con vert ing energy
from one form to another.
Joule was deeply re li gious, and it seemed right to him that there should be a unity for
all the forces of na ture. He sus pected that heat was some how the ul ti mate and nat u ral form
for cal cu lat ing the equiv a lence of dif fer ent forms of energy.
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Joule turned his at ten tion to the con ver sion of me chan i cal en ergy into heat. In real life
a mov ing body (with the me chan i cal en ergy of mo men tum) even tu ally stopped. What hap -
pened to its en ergy? He de signed a se ries of ex per i ments us ing wa ter to mea sure the con ver -
sion of me chan i cal motion into heat.
Two of Joule’s ex per i ments be came fa mous. First, he sub merged an air-filled cop per
cyl in der in a tub of wa ter and mea sured the wa ter tem per a ture. He then pumped air into the
cyl in der un til it reached 22 at mo spheres of pres sure. The gas law said that the me chan i cal
work to cre ate this in creased air pres sure should cre ate heat. But would it? Joule mea sured a
0.285°F rise in wa ter tem per a ture. Yes, me chan i cal energy had been converted to heat.
Next, Joule at tached pad dles onto a ver ti cal shaft that he low ered into a tub of wa ter.
Fall ing weights (like on a grand fa ther clock) spun the pad dles through the tub’s wa ter. This
me chan i cal ef fort should be par tially con verted to heat. But was it?
His re sults were in con clu sive un til Joule switched from wa ter to liq uid mer cury. With
this denser fluid, he eas ily proved that the me chan i cal ef fort was con verted to heat at a fixed
rate. Liq uid was heated by merely stir ring it.
Joule re al ized that all forms of en ergy could be con verted into equiv a lent amounts of
heat. He pub lished these re sults in 1843 and in tro duced stan dard heat en ergy units to use for
cal cu lat ing these equiv a lences. Since then, phys i cists and chem ists typ i cally use these units
and have named them joules. Bi ol o gists pre fer to use an al ter nate unit called the cal o rie
(4.18 joules = 1 cal o rie). With this dis cov ery that any form of en ergy could be con verted
into an equiv a lent amount of heat en ergy, Joule pro vided a way to ad vance the study of
energy, mechanics, and technologies.
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