The kinetic theory of matter assumes that all matter is made up of tiny,
ultimate particles of matter called molecules. A molecule is defined as
the smallest particle of a compound or a gaseous element that can
exist and still retain the characteristic properties of that substance. Molecules
interact, attracting each other through a force of cohesion. Liquids,
solids, and gases are the phases of matter that are explained by the
molecular arrangements and forces of attraction between their molecules.
A solid has a definite shape and volume because it has molecules
that vibrate in a fixed equilibrium position with strong cohesive forces.
A liquid has molecules that have cohesive forces strong enough to give
it a definite volume but not strong enough to give it a definite shape.
The molecules of a liquid can flow, rolling over each other. A gas is composed
of molecules that are far apart, with weak cohesive forces. Gas molecules move freely in a constant, random motion.
The temperature of an object is related to the average kinetic energy
of the molecules making up the object. A measure of temperature tells
how hot or cold an object is on two arbitrary scales, the Fahrenheit scale
and the Celsius scale. The absolute scale, or Kelvin scale, has the coldest temperature possible (-273°C) as zero (0 K).
The observable potential and kinetic energy of an object is the external
energy of that object, while the potential and kinetic energy of the
molecules making up the object is the internal energy of the object. Heat
refers to the total internal energy and is a transfer of energy that takes
place (1) because of a temperature difference between two objects or
(2) because of an energy-form conversion. An energy-form conversion is
actually an energy conversion involving work at the molecular level, so all
energy transfers involve heating and working.
A quantity of heat can be measured in joules (a unit of work or energy)
or calories (a unit of heat). A kilocalorie is 1,000 calories, another
unit of heat. A Btu, or British thermal unit, is the English system unit of
heat. The mechanical equivalent of heat is 4,184 J = 1 kcal.
The specific heat of a substance is the amount of energy (or heat)
needed to increase the temperature of 1 gram of a substance 1 degree Celsius.
The specific heat of various substances is not the same because the
molecular structure of each substance is different.
Energy transfer that takes place because of a temperature difference
does so through conduction, convection, or radiation. Conduction
is the transfer of increased kinetic energy from molecule to molecule.
Substances vary in their ability to conduct heat, and those that are poor
conductors are called insulators. Gases, such as air, are good insulators.
The best insulator is a vacuum. Convection is the transfer of heat by the
displacement of large groups of molecules with higher kinetic energy.
Convection takes place in fluids, and the fluid movement that takes
place because of density differences is called a convection current. Radiation
is radiant energy that moves through space. All objects with an absolute
temperature above zero give off radiant energy, but all objects
absorb it as well. Energy is transferred from a hot object to a cold one
through radiation.
The transition from one phase of matter to another is called a phase
change. A phase change always absorbs or releases a quantity of latent heat
not associated with a temperature change. Latent heat is energy that goes
into or comes out of internal potential energy. The latent heat of fusion is
absorbed or released at a solid-liquid phase change. The latent heat of fusion
for water is 80.0 cal/g (144.0 Btu/lb). The latent heat of vaporization
is absorbed or released at a liquid-gas phase change. The latent heat of vaporization
for water is 540.0 cal/g (970.0 Btu/lb).
Molecules of liquids sometimes have a high enough velocity to escape
the surface through the process called evaporation. Evaporation is a
cooling process, since the escaping molecules remove the latent heat of
vaporization in addition to their high molecular energy.Vapor molecules
return to the liquid state through the process called condensation. Condensation
is the opposite of evaporation and is a warming process.When
the condensation rate equals the evaporation rate, the air is said to be
saturated. The rate of evaporation can be increased by (1) increased temperature,
(2) increased surface area, (3) removal of evaporated molecules, and (4) reduced atmospheric pressure.
Warm air can hold more water vapor than cold air, and the ratio of
how much water vapor is in the air to how much could be in the air at
that temperature (saturation) is called relative humidity.Thermodynamics is the study of heat and its relationship to mechanical
energy, and the laws of thermodynamics describe these relationships:
The first law of thermodynamics is a thermodynamic statement of
the law of conservation of energy. The second law of thermodynamics
states that heat flows from objects with a higher temperature to objects
with a cooler temperature. The second law implies a degradation of energy
as high-quality (more ordered) energy sources undergo degradation
to low-quality (less ordered) sources. Entropy is a thermodynamic measure
of disorder, and entropy is seen as continually increasing in the universe
and may result in the maximum disorder called the heat death of the universe.
