In thermodynamics, heat is defined as a form of energy that is transferred between a system and its surroundings solely because of a temperature difference. Heat flows spontaneously from a region of higher temperature to a region of lower temperature and continues until thermal equilibrium is attained.
Heat is not a property of a system but a mode of energy transfer that occurs only during a thermodynamic process. Once the transfer of heat is complete, the system does not retain heat as a stored quantity; instead, the transferred energy contributes to a change in the internal energy of the system.
Heat as a Path Function
The amount of heat exchanged during a process depends on the path by which the process occurs. For this reason, heat is classified as a path function. If a system changes from an initial state to a final state through different paths, the quantity of heat exchanged may be different for each path even though the initial and final states are identical.
Since heat is a path function, it cannot be expressed as a function of state variables alone, and its differential is written as an inexact differential, represented by the symbol δq.
Sign Convention for Heat
To apply thermodynamic equations consistently, a sign convention is adopted for heat. When heat is supplied to the system from the surroundings, it is taken as positive and denoted by q > 0. When heat is released by the system to the surroundings, it is taken as negative and denoted by q < 0.
This sign convention ensures compatibility with the mathematical form of the First Law of Thermodynamics and provides a uniform method for energy accounting.
Heat and Temperature
Although heat and temperature are closely related, they are fundamentally different concepts. Temperature is a measure of the thermal state of a system and indicates the tendency of heat to flow, whereas heat is the energy transferred as a result of a temperature difference.
Two systems at the same temperature do not exchange heat when brought into contact, whereas systems at different temperatures exchange heat until thermal equilibrium is achieved.
Modes of Heat Transfer
Heat may be transferred from one system to another through three distinct modes: conduction, convection, and radiation. In conduction, heat is transferred through a material medium without any bulk movement of matter. In convection, heat transfer occurs due to the bulk motion of a fluid. In radiation, heat is transferred in the form of electromagnetic waves and does not require a material medium.
Thermodynamics is primarily concerned with the amount of heat transferred and its effect on the state of the system rather than the detailed mechanisms by which heat transfer occurs.
Heat in the First Law of Thermodynamics
According to the First Law of Thermodynamics, heat is one of the two primary modes by which energy crosses the boundary of a system. When heat is supplied to a system, it may increase the internal energy of the system, perform work, or contribute to both.
\[\Delta U = q – W\]
This equation shows that heat plays a central role in determining changes in internal energy and must always be considered along with work when analyzing thermodynamic processes.
Having understood heat as a mode of energy transfer, we now proceed to study work, the second mode by which energy is exchanged between a system and its surroundings.