In heat transfer, conduction (or heat conduction) is the
transfer of heat energy by microscopic diffusion and collisions
of particles or quasi-particles within a body due to a
temperature gradient. The microscopically diffusing and
colliding objects include molecules, electrons, atoms, and
phonons. They transfer microscopically disorganized kinetic and
potential energy, which are jointly known as internal energy.
Conduction takes place in all forms of ponderable matter, such
as solids, liquids, gases and plasmas. By conduction, as well as
by thermal radiation, heat spontaneously flows from a body at a
higher temperature to a body at a lower temperature. In the
absence of external drivers, temperature differences decay over
time, and the bodies approach thermal equilibrium. In
conduction, heat flows within and through the body itself. In
contrast, in heat transfer by thermal radiation, the transfer is
often between bodies. Also possible is transfer of heat by a
combination of conduction and thermal radiation.