When heat is transformed into a form of energy, or when other forms of energy are transformed into heat, total amounts of energy (heat plus other forms) in the system is constant. This is known as the first law of thermodynamics, i.e., the conservation of energy. To express it another way: it is in no way possible either by mechanical, thermal, chemical, or other means, to obtain a perpetual motion machine; i.e., one that creates its own energy.

throughout the chemical industries, some general concepts and definitions regarding the subject of heat transfer are reviewed. Heat in physics, refers to the transfer of energy from one part of a substance to another, or from one object to another, because of a difference of temperature. Heat flows from a substance of higher temperature to a substance at a colder temperature, provided the volume of the objects remains constant. Heat does not flow from a lower to a much higher temperature, unless other forms of energy transfer, work, is also present. Until the beginning of the 19th century, it was thought that heat was an invisible substance called caloric. An object at a high temperature was thought to contain caloric more than one at a low temperature. However, British physicist Benjamin Thompson in 1798 and British chemist Sir Humphry Davy in 1799 presented evidence that heat exchange, like work, is a form of energy transfer. In a series of experiments between 1840 and 1849, British physicist James Prescott Joule provided conclusive evidence that heat is a form of energy in transit, and that it can cause the same changes as work.

A steam engine uses a source of heat to produce a type of work. Is it possible to completely convert the heat, making it a 100% efficient machine? The answer is to be found in the second law of thermodynamics: No cyclic machine can convert heat energy wholly into other types of heat exchange or energy. It is not possible to construct a cyclic machine that does nothing, but withdraw heat energy and convert it into mechanical energy. The second law of thermodynamics implies the irreversibility of certain processes - that of converting all heat into mechanical energy, although it is possible to have a cyclic machine that does nothing but convert mechanical energy or into heat.

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