Heat energy is most intense in
substances whose molecules are moving rapidly in a very disorderly way. Such a
substance will give up some of its heat to another substance whose molecules
are less agitated. When this happens, the heat is said to “flow” from one
substance to another (or from one body to another). The energy transfer is
indicated by a change in temperature.
Temperature, therefore, is not the same
thing as heat—although the two words are often used interchangeably.
Temperature can be defined as the degree of intensity of hotness or coldness.
“Hotness” and “coldness,” however, are comparative terms. A flame, for example,
is hot when compared with ice but cold when compared with the sun. This
definition of temperature, therefore, is vague and unscientific, although it
does convey the correct impression that temperature is a measure of relative
intensity rather than of quantity.
A more specific definition is:
temperature is the ability of one body to give up heat energy to another body.
A hot body becomes cooler, and a cold body becomes warmer, as long as heat is
flowing from one to the other. The hot body has a greater ability to give up
heat and therefore has a higher temperature. After a time the two bodies may
reach a condition of heat equilibrium, or balance of heat intensity. Then, heat
flow ceases. At the point of equilibrium both bodies can be said to be at the
same temperature.
Measurement
of Temperature
Temperature is measured by means of
instruments called thermometers. Several temperature scales have been devised
for relating the hotness and coldness of bodies to fixed temperatures, such as
the freezing point and boiling point of water. On most temperature scales, the
unit of temperature is called a degree. The Kelvin scale is an exception; its
unit of temperature is the kelvin.
The Fahrenheit, Celsius (or centigrade),
and Reaumur scales are used in the range of temperatures important for human
comfort, laboratory experiments, and industrial processes.
The Rankine scale and the Kelvin scale
are based on the concept of absolute zero; all temperature readings on these
scales are positive numbers. The Kelvin scale is widely used in scientific
work. The Rankine scale is used primarily by British and American engineers.
Absolute
Zero
Experiments have shown that every 1° C.
increase or decrease in temperature causes the pressure exerted by a gas to
increase or decrease at the constant rate of 1/273.15 of its pressure at 0° C.
This means that at -273.15° C. an ideal (theoretical) gas would exert no
pressure at all. Since experiments with real gases have shown a clear relation
between pressure and temperature, zero pressure would indicate that the ideal
gas had lost all its ability to give up heat. Its molecules would be absolutely
motionless. This is impossible—molecules are always agitated, to some
extent—and therefore the absolute zero of temperature remains a theoretical
concept. The concept is, however, a useful one, for it gives a base point to
which all temperature measurements may be referred, in positive numbers.
The idea that absolute zero can never be
reached is sometimes considered important enough to be called the third law of
thermodynamics. Scientists have succeeded in cooling substances to within a
small fraction of a degree above absolute zero. The study of the behavior of
substances at very low temperatures is called cryogenics.
High
Temperatures
Absolute zero is the lower limit for
temperature, but there is no upper limit. The hottest substances known are
ionized gases in certain stars, with temperatures of a billion degrees or more.
Measurement
of Heat
The heat released or absorbed in a
physical or chemical process can be measured with an instrument called a
calorimeter. Commonly used units for measuring heat are the calorie and the
British thermal unit, or Btu. Heat is also measured in such other units as the
joule (the unit of energy in the SI, or metric system).
Heat travels by conduction, convection,
or radiation, or a combination of these methods.
Conduction
As molecules move about they frequently
bump into each other. According to the second law of thermodynamics, the
faster-moving (hotter) molecules will give up some of their heat energy to the
slower-moving (colder) molecules whenever collisions occur. The newly heated
molecules then will be able to pass on a share of heat to molecules possessing
less heat. The process continues in the direction away from the hottest
molecules. By this means, heat is conducted (led) from the warm to the cool
parts of a substance, or into a cool body that is in contact with a warm body.
The heating of an iron rod, as shown in the drawing How Heat Travels,
illustrates both: the heat spreads through the rod and also warms the handle
that holds the rod.
Substances vary in their ability to
conduct heat. Air and water are rather poor conductors. Most metals conduct heat
rapidly. Asbestos conducts heat so poorly that it is used as a heat insulator.
Convection
When a fluid (a liquid or gas) is
heated, the portion of the fluid nearest the heat source will expand as it
gains energy. In expanding, this portion becomes less dense (lighter) and is
pushed upward by cooler, heavier portions of the surrounding fluid. The
displacement brings the cooler portions nearer the heat source, and they in
turn gain energy, become lighter, and are pushed upward. The resulting
movements, or currents (called convection currents), distribute heat from the
source throughout the fluid.
Heat will continue to travel by
convection as long as temperature differences exist within the fluid. Examples
of convection are the movement of warm air in a room and the circulation of
water in a kettle placed over a fire.
Radiation
All bodies continually give off energy
in the form of rays. The rays may be composed of particles or waves. Heat rays,
called infrared radiation, are electromagnetic waves that resemble light waves
but have somewhat longer wavelengths.
A body emits heat rays as a result of
the vibration of its molecules. As the rays are emitted, the molecules lose
some of their energy. When another body absorbs the rays, its molecules become
more agitated and the body thus gains heat energy. Heat rays can travel through
a vacuum. Infrared radiation from the sun, for example, passes through empty
space to reach the earth.
The amount of radiation a body will
absorb varies with the material out of which it is made and the nature of its
surface. In general, materials with a dark or rough surface will absorb more
infrared radiation than materials with a white or shiny surface.
The main changes that substances undergo
when they are heated are (1) increase in temperature, (2) change of state, and
(3) expansion. Each of these changes depends on properties that differ from one
substance to another. The rate of temperature change depends on the specific
heat of the substance. Change of state—from solid to liquid, or from liquid to
gas—occurs only when the latent heat requirements of the substance have been
met. Expansion of solids and liquids takes place in accordance with the
coefficient of expansion of the substance.
Specific
Heat
The amount of heat required to raise the
temperature of a unit mass of a substance by a certain amount is called that
substance's specific heat. It is expressed as a ratio to the specific heat of
water, which by definition is 1.
Latent
Heat
The amount of heat energy that must be
absorbed or released by a given quantity of a substance to bring about a
complete change of state in the substance. As the latent heat is absorbed or
released, the temperature of the substance remains the same. Latent heat is
usually referred to in terms of the type of change of state involved.
Heat of Fusion is the latent heat needed
to change a substance from a solid to a liquid. The change is called melting.
Heat of Vaporization is the latent heat
needed to change a substance from a liquid to a gas (vapor). The change is
called boiling if the vapor forms within the liquid, or evaporation if the
vapor forms only at the surface.
Heat of Sublimation. At atmospheric
pressure some substances when heated change directly from a solid to a gas.
This change of state is called sublimation, and the latent heat needed for it
is called the heat of sublimation.
Heat of Condensation is the latent heat
given up by a substance in changing from a gas to a liquid. The change of state
is sometimes called liquefaction. For any given substance, the heat of
condensation is equal to the heat of vaporization.
Heat of Solidification is the latent
heat given up by a substance in changing from a liquid to a solid or, for those
substances that undergo sublimation, in changing from a gas into a solid. In
the first case, the heat of solidification is equal to the heat of fusion and
the change of state is commonly called freezing. In the second case, the heat
of solidification is equal to the heat of sublimation.
Expansion
As a substance gains heat energy, its
molecules push farther apart, causing the substance to occupy more space. This
increase in size is called expansion. Solids and liquids expand in the same
way. Gases are subject to different laws.
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