Temperature is an indicator of the presence of thermal
energy: when the temperature of an objects increases,
energy has been added to it.
Many physical properties depend on temperature. The
most common kinds of thermometers make use of thermal
expansion.
Objects placed in contact will exchange energy, and
when isolated from other sources of energy will come to have
the same temperature.
The important ways to move thermal energy from one place to
another are
thermal conduction. Metals are good thermal conductors, while
air is a very poor thermal conductor.
convection, which involves motion of a gas or liquid
radiation of light (including invisible infrared light)
evaporation and condensation (of water)
It takes energy to cause a change of phase (solid into liquid, or
liquid into gas), without having any change in temperature at all.
The amount of energy is large compared to the amount of energy
required to change the temperature of the object by a few
degrees.
Assuming that there is no change of phase involved, the
amount of energy required to change the temperature of
an object is proportional to the amount of stuff in the object
(mass or number of atoms), and to the change in temperature.
It also depends on the substance of which the object is made.
There are two fundamental laws of physics governing heat:
The First Law of Thermodynamics, which says that the total energy
of the universe is constant
The Second Law of Thermodynamics, which has several implications:
Thermal energy will move spontaneously from a hot object
to a cooler one, and never the other way.
Systems progress from order to disorder, but will not spontaneously
become more ordered.