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An isothermal process is a type of thermodynamic process in which the temperature T of a system remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a change in the system occurs slowly enough to allow the system to be continuously adjusted to the temperature of the reservoir through heat exchange (see quasi-equilibrium).
Work and heat are not thermodynamic properties, but rather process quantities: flows of energy across a system boundary. Systems do not contain work, but can perform work, and likewise, in formal thermodynamics, systems do not contain heat, but can transfer heat.
Thermodynamic work is one of the principal kinds of process by which a thermodynamic system can interact with and transfer energy to its surroundings. This results in externally measurable macroscopic forces on the system's surroundings, which can cause mechanical work, to lift a weight, for example, [1] or cause changes in electromagnetic, [2] [3] [4] or gravitational [5] variables.
The system always contains the same amount of matter, but (sensible) heat and (boundary) work can be exchanged across the boundary of the system. Whether a system can exchange heat, work, or both is dependent on the property of its boundary. Adiabatic boundary – not allowing any heat exchange: A thermally isolated system
In other words, the system is dynamically connected, by a movable boundary, to a constant-pressure reservoir. An isochoric process is one in which the volume is held constant, with the result that the mechanical PV work done by the system will be zero. On the other hand, work can be done isochorically on the system, for example by a shaft that ...
In thermodynamics, a diathermal wall between two thermodynamic systems allows heat transfer but does not allow transfer of matter across it.. The diathermal wall is important because, in thermodynamics, it is customary to assume a priori, for a closed system, the physical existence of transfer of energy across a wall that is impermeable to matter but is not adiabatic, transfer which is called ...
and thus for a system kept at constant temperature and volume and not capable of performing electrical or other non-PV work, the total free energy during a spontaneous change can only decrease. This result seems to contradict the equation d F = − S d T − P d V , as keeping T and V constant seems to imply d F = 0, and hence F = constant.
Along with the idea of an adiabatic wall is that of an adiabatic enclosure. It is easily possible that a system has some boundary walls that are adiabatic and others that are not. When some are not adiabatic, then the system is not adiabatically enclosed, though adiabatic transfer of energy as work can occur across the adiabatic walls.