Page 228 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 18

Heat

INTERNAL ENERGY

Every body of matter, whether solid, liquid, or gas, consists of atoms or molecules in rapid motion. The kinetic
energies of these particles constitute the internal energy of the body of matter. The temperature of the body is
a measure of the average kinetic energy of its particles. Heat may be thought of as internal energy in transit.
When heat is added to a body, its internal energy increases and its temperature rises; when heat is removed from
a body, its internal energy decreases and its temperature falls.

TEMPERATURE

Temperature is familiar as the property of a body of matter responsible for sensations of hot or cold to the touch.
Temperature indicates the direction of internal energy flow: When two objects are in contact, internal energy goes
from the one at higher temperature to the one at lower temperature, regardless of the total amounts of internal
energyineach.Thusifhotcoffeeispouredintoacoldcup,thecoffeebecomescoolerandthecupbecomeswarmer.
A thermometer is a device for measuring temperature. Matter usually expands when heated and contracts
when cooled, the relative amount of change being different for different substances. This behaviour is the basis
of most thermometers, which make use of the different rates of expansion of mercury or alcohol and glass, or of
two joined metal strips, to indicate temperature.

TEMPERATURE SCALES
◦
The Celsius (or centrigrade) temperature scale assigns 0 C to the freezing point of water and 100 C to its boiling
◦
◦
point. On the Fahrenheit scale these points are, respectively, 32 and 212 F. A Fahrenheit degree is therefore
five-ninths as large as a Celsius degree. The following formulas give the procedure for converting a temperature
expressed in one scale to the corresponding value in the other:
9
T F = T C + 32 ◦
5
5
◦
T C = (T F − 32 )
9
SOLVED PROBLEM 18.1
◦
What is the Celsius equivalent of 80 F?
5
5
T C = (T F − 32 ) = (80 − 32 ) = 26.7 C
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