In
physics,
heat is
energy as it spontaneously passes between a system and its surroundings, other than as
work or with the transfer of matter. In
thermodynamics, finer detail of the process of transfer is in principle unspecified. When there is a suitable physical pathway, heat transfer occurs from a hotter to a colder body. The transfer can be direct, by contact, from the source to the destination body, as in
conduction combined with
radiation, or remote, as in radiation without conduction; or indirect, by conduction and radiation through a thick solid wall, or by way of an intermediate fluid body, as in
convective circulation.
