Heat is a transfer of energy due to a difference of temperature. Work is a transfer of energy by mechanical means, not due to a temperature difference. Thermodynamics studies the relation between heat and work. A practical application is the generation of work from heat which is the most common means of 'producing' energy worldwide. Thermodynamics is based on three empirical laws (axioms).
The First Law of Thermodynamics
The First Law of Thermodynamics states the conservation of energy in a closed system: the change in internal energy of a closed system, ΔU will be equal to the heat Q added to the system minus the work W done by the system on the surroundings.
A closed system in thermodynamics does not allow input or output of mass. It allows input and output of energy in the form of heat or work such that the Eq. (1) holds true.
The following sign convention is adopted:
Q > 0: heat flowing into the system
Q < 0: heat flowing out of the system
W > 0: work done by the system
W > 0: work done on the system
The Second Law of Thermodynamics
The second law of thermodynamics is a statement about which processes occur in nature and which do not. It can be stated in a variety of ways, all of which are equivalent.
Heat cannot flow spontaneously from a cold object to a hot object (R. J. E. Clausius, 1822–1888). Notice, that the conservation of energy (First Law) does not exclude the possibility of heat flow from a cold to to a warm object.
No cyclic device is possible which transforms heat completely into work (Kelvin-Planck formulation).
No device is possible whose sole effect is to transfer heat from one system at a lower temperature into a second system at a higher temperature (Clausius).
The second law of thermodynamics is closely related to the concept of entropy. Although entropy S is a state variable of a system not its absolute value is considered but the change in entropy ΔS by adding heat Q is (by a reversible process) at constant temperature T:
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