# Man's physical universe

ENERGY MANIFESTS ITSELF IN MANY FORMS 293

reaction is called oxidation. The oxidation of the foods in our bodies

produces body heat and other forms of energy. The oxidation of coal,

wood, petroleum, and natural gas produces most of the heat used to

run our machinery and to heat and light our homes.

Heat is produced by electricity. Most electric heating devices produce

heat by passing the electric current through wire which offers

high resistance to the passage of the current and therefore becomes hot.

from the sun is, of course, our ultimate source of heat. It is more convenient

to change radiant energy into chemical energy in the form of

food or fuel and then burn the substances so formed, than it is to

transform light directly into heat.

Heat Is Measured by Means of Calorimeters.

The calorie is the amount of heat required to raise the temperature of

one gram of water one degree centigrade. A calorimeter is simply a

vessel in which we measure the change in the temperature of a given

weight of water by the application of heat under such conditions that

but little heat is lost by radiation.

A thermos bottle makes an excellent calorimeter for the measurement

of the specific heats of metals or the heat evolved by certain

chemical reactions. The product of a change in temperature and the

number of grams of water undergoing the change gives the number

of small calories. The large Calorie, written with a capital C, is equal

to 1000 small calories.

The calorie is used in most scientific measurements, but the quantity

factor of heat still used by engineers in the English-speaking countries

is the B.T.U. (British thermal unit), which is the amount of heat

required to raise

the temperature of one pound of water one degree

Fahrenheit.

Inasmuch as coal is bought for its heating value, many consumers

of large quantities pay for it on the basis of its calorific value. Coal,

fuels, foods, and other organic substances may be burned in compressed

oxygen gas in a bomb calorimeter to determine their calorific

values.

Large, roomlike calorimeters have been employed to measure the

number of calories used up in sleeping, thinking, exercising, etc.

Thermodynamics Deals with the Conversion of Heat into Mechanical

Energy and Vice Versa.

The First Law of Thermodynamics. The first law of thermodynamics,

stated non-technically, is: "Something cannot be obtained

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