HEAT TRANSFER TERMINOLOGY
Heat and Work
Distinction should also be made between the energy terms heat and work. Both represent energy
in transition. Work is the transfer of energy resulting from a force acting through a distance.
Heat is energy transferred as the result of a temperature difference. Neither heat nor work are
thermodynamic properties of a system. Heat can be transferred into or out of a system and work
can be done on or by a system, but a system cannot contain or store either heat or work. Heat
into a system and work out of a system are considered positive quantities.
When a temperature difference exists across a boundary, the Second Law of Thermodynamics
indicates the natural flow of energy is from the hotter body to the colder body. The Second Law
of Thermodynamics denies the possibility of ever completely converting into work all the heat
supplied to a system operating in a cycle. The Second Law of Thermodynamics, described by
Max Planck in 1903, states that:
It is impossible to construct an engine that will work in a complete cycle and
produce no other effect except the raising of a weight and the cooling of a
The second law says that if you draw heat from a reservoir to raise a weight, lowering the weight
will not generate enough heat to return the reservoir to its original temperature, and eventually
the cycle will stop. If two blocks of metal at different temperatures are thermally insulated from
their surroundings and are brought into contact with each other the heat will flow from the hotter
to the colder. Eventually the two blocks will reach the same temperature, and heat transfer will
cease. Energy has not been lost, but instead some energy has been transferred from one block
Modes of Transferring Heat
Heat is always transferred when a temperature difference exists between two bodies. There are
three basic modes of heat transfer:
Conduction involves the transfer of heat by the interactions of atoms or molecules of a
material through which the heat is being transferred.
Convection involves the transfer of heat by the mixing and motion of macroscopic
portions of a fluid.
Radiation, or radiant heat transfer, involves the transfer of heat by electromagnetic
radiation that arises due to the temperature of a body.
The three modes of heat transfer will be discussed in greater detail in the subsequent chapters
of this module.