Distinction of Thermal Energy and Heat
In thermodynamics, heat must always be defined as energy in exchange between two systems, or a single system and its surroundings. According to the zeroth law of thermodynamics, heat is exchanged between thermodynamic systems in thermal contact only if their temperatures are different, as this is the condition when the net exchange of thermal energy is non-zero. For the purpose of distinction, a system is defined to be enclosed by a well-characterized boundary. If heat traverses the boundary in direction into the system, the internal energy change is considered to be a positive quantity, while exiting the system, it is negative. As a process variable, heat is never a property of the system, nor is it contained within the boundary of the system.
In contrast to heat, thermal energy exists on both sides of a boundary. It is the statistical mean of the microscopic fluctuations of the kinetic energy of the systems' particles, and it is the source and the effect of the transfer of heat across a system boundary. Statistically, thermal energy is always exchanged between systems, even when the temperatures on both sides is the same, i.e. the systems are in thermal equilibrium. However, at equilibrium, the net exchange of thermal energy is zero, and therefore there is no heat.
Thermal energy may be increased in a system by other means than heat, for example when mechanical or electrical work is performed on the system. No qualitative difference exists between the thermal energy added by other means. Thermal energy is a state function. There is also no need in classical thermodynamics to characterize the thermal energy in terms of atomic or molecular behavior. A change in thermal energy induced in a system is the product of the change in entropy and the temperature of the system.
Heat exchanged across a boundary may cause changes other than a change in thermal energy. For example, it may cause phase transitions, such as melting or evaporation, which are changes in the configuration of a material. Since such an energy exchange is not observable by a change in temperature, it is called a latent heat and represents a change in the potential energy of the system.
Rather than being itself the thermal energy involved in a transfer, heat is sometimes also understood as the process of that transfer, i.e. heat functions as a verb.
Today's narrow definition of heat in physics contrasts with its use in common language, in some engineering disciplines, and in the historical scientific development of thermodynamics in the caloric theory of heat. The phenomenon of heat in these instances is today properly identified as the entropy.
Read more about this topic: Thermal Energy
Famous quotes containing the words distinction of, distinction, energy and/or heat:
“Met face to face, these Indians in their native woods looked like the sinister and slouching fellows whom you meet picking up strings and paper in the streets of a city. There is, in fact, a remarkable and unexpected resemblance between the degraded savage and the lowest classes in a great city. The one is no more a child of nature than the other. In the progress of degradation the distinction of races is soon lost.”
—Henry David Thoreau (18171862)
“Only where there is pecuniary equality can the distinction of merit stand out.”
—George Bernard Shaw (18561950)
“Because humans are not alone in exhibiting such behaviorbees stockpile royal jelly, birds feather their nests, mice shred paperits possible that a pregnant woman who scrubs her house from floor to ceiling [just before her baby is born] is responding to a biological imperative . . . . Of course there are those who believe that . . . the burst of energy that propels a pregnant woman to clean her house is a perfectly natural response to their mothers impending visit.”
—Mary Arrigo (20th century)
“Two wooden tubs of blue hydrangeas stand at the foot of the stone steps.
The sky is a blue gum streaked with rose. The trees are black.
The grackles crack their throats of bone in the smooth air.
Moisture and heat have swollen the garden into a slum of bloom.
Pardie! Summer is like a fat beast, sleepy in mildew....”
—Wallace Stevens (18791955)