is a number that is related to the average kinetic energy
of the molecules of a substance. If temperature is measured
in Kelvin degrees, then this number is directly proportional
to the average kinetic energy of the molecules.
is a measurement of the total energy in a substance. That
total energy is made up of not only of the kinetic energies
of the molecules of the substance, but total energy is also
made up of the potential energies of the molecules.
temperature is not energy. It is, though, a number that
relates to one type of energy possessed by the molecules
of a substance. Temperature directly relates to the kinetic
energy of the molecules. The molecules have another type
of energy besides kinetic, however; they have potential
energy, also. Temperature readings do not tell you anything
directly about this potential energy.
can be measured in a variety of units. If you measure it
in degrees Kelvin, then the temperature value is directly
proportional to the average kinetic energy of the molecules
in the substance. Notice we did not say that temperature
is the kinetic energy. We said it is a number, if in degrees
Kelvin, is proportional to the average kinetic energies
of the molecules; that is, if you double the Kelvin temperature
of a substance, you double the average kinetic energy of
is the total amount of energy possessed by the molecules
in a piece of matter. This energy is both kinetic energy
and potential energy.
heat, (i. e., energy), goes into a substance one of two
things can happen:
1. The substance can experience a raise in temperature.
That is, the heat can be used to speed up the molecules
of the substance. Since Kelvin temperature is directly proportional
to the average kinetic energy of molecules in a substance,
an factor increase in temperature causes an equal factor
increase in the average kinetic energy of the molecules.
And if the kinetic energy of the molecules increase, the
speed of the molecules will increase, although these increases
are not directly proportional. The kinetic energy of a body
is proportional to the square of the speed of the body.
2. The substance can change state. For example, if the substance
is ice, it can melt into water. Perhaps surprisingly, this
change does not cause a raise in temperature. The moment
before melting the average kinetic energy of the ice molecules
is the same as the average kinetic energy of the water molecules
a moment after melting. Although heat is absorbed by this
change of state, the absorbed energy is not used to speed
up the molecules. The energy is used to change the bonding
between the molecules. Changing the manner in which the
molecules bond to one another constitutes a change in potential
energy. Heat comes in and there is an increase in the potential
energy of the molecules. Their kinetic energy remains unchanged.
when heat comes into a substance, energy comes into a substance.
That energy can be used to increase the kinetic energy of
the molecules, which would cause an increase in temperature.
Or that heat could be used to increase the potential energy
of the molecules causing a change in state that is not accompanied
by an increase in temperature.