_Unit_ _Water_ _Temperature Rise_
1 B. t. u. 1 Pound 1 Degree Fahrenheit
1 Calorie 1 Kilogram 1 Degree centigrade
But 1 kilogram = 2.2046 pounds and 1 degree centigrade = 9/5 degree
Fahrenheit.
Hence 1 calorie = (2.2046 × 9/5) = 3.968 B. t. u.
The heat values in B. t. u. are ordinarily given per pound, and the heat
values in calories per kilogram, in which case the B. t. u. per pound
are approximately equivalent to 9/5 the calories per kilogram.
As determined by Joule, heat energy has a certain definite relation to
work, one British thermal unit being equivalent from his determinations
to 772 foot pounds. Rowland, a later investigator, found that 778 foot
pounds were a more exact equivalent. Still later investigations indicate
that the correct value for a B. t. u. is 777.52 foot pounds or
approximately 778. The relation of heat energy to work as determined is
a demonstration of the first law of thermo-dynamics, namely, that heat
and mechanical energy are mutually convertible in the ratio of 778 foot
pounds for one British thermal unit. This law, algebraically expressed,
is W = JH; W being the work done in foot pounds, H being the heat in
B. t. u., and J being Joules equivalent. Thus 1000 B. t. u.'s would be
capable of doing 1000 × 778 = 778000 foot pounds of work.
Specific Heat--The specific heat of a substance is the quantity of heat
expressed in thermal units required to raise or lower the temperature of
a unit weight of any substance at a given temperature one degree. This
quantity will vary for different substances For example, it requires
about 16 B. t. u. to raise the temperature of one pound of ice 32
degrees or 0.5 B. t. u. to raise it one degree, while it requires
approximately 180 B. t. u. to raise the temperature of one pound of
water 180 degrees or one B. t. u. for one degree.
If then, a pound of water be considered as a standard, the ratio of the
amount of heat required to raise a similar unit of any other substance
one degree, to the amount required to raise a pound of water one degree
is known as the specific heat of that substance. Thus since one pound of
water required one B. t. u. to raise its temperature one degree, and one
pound of ice requires about 0.5 degrees to raise its temperature one
degree, the ratio is 0.5 which is the specific heat of ice. To be exact,
the specific heat of ice is 0.504, hence 32 degrees × 0.504 = 16.128
B. t. u. would be required to raise the temperature of one pound of ice
from 0 to 32 degrees. For solids, at ordinary temperatures, the specific
heat may be considered a constant for each individual substance,
although it is variable for high temperatures. In the case of gases a
distinction must be made between specific heat at constant volume, and
at constant pressure.
Where specific heat is stated alone, specific heat at ordinary
temperature is implied, and _mean_ specific heat refers to the average
value of this quantity between the temperatures named.
The specific heat of a mixture of gases is obtained by multiplying the
specific heat of each constituent gas by the percentage by weight of
that gas in the mixture, and dividing the sum of the products by 100.
The specific heat of a gas whose composition by weight is CO_{2}, 13 per
cent; CO, 0.4 per cent; O, 8 per cent; N, 78.6 per cent, is found as
follows:
CO_{2} : 13 × 0.217 = 2.821
CO : 0.4 × 0.2479 = 0.09916
O : 8 × 0.2175 = 1.74000
N : 78.6 × 0.2438 = 19.16268
--------
100.0 23.82284
and 23.8228 ÷ 100 = 0.238 = specific heat of the gas.
The specific heats of various solids, liquids and gases are given in
Table 4.
Sensible Heat--The heat utilized in raising the temperature of a body,
as that in raising the temperature of water from 32 degrees up to the
boiling point, is termed sensible heat. In the case of water, the
sensible heat required to raise its temperature from the freezing point
to the boiling point corresponding to the pressure under which
ebullition occurs, is termed the heat of the liquid.
Latent Heat--Latent heat is the heat which apparently disappears in
producing some change in the condition of a body without increasing its
temperature If heat be added to ice at freezing temperature, the ice
will melt but its temperature will not be raised. The heat so utilized
in changing the condition of the ice is the latent heat and in this
particular case is known as the latent heat of fusion. If heat be added
to water at 212 degrees under atmospheric pressure, the water will not
become hotter but will be evaporated into steam, the temperature of
which will also be 212 degrees. The heat so utilized is called the
latent heat of evaporation and is the heat which apparently disappears
in causing the substance to pass from a liquid to a gaseous state.