Energy at 20 degrees Celsius provides a comfortable living environment, but is essentially useless for producing mechanical work in everyday situations. Thus a kilogram of steam at 1,000 degrees Celsius will produce more mechanical energy than steam at 500 degrees Celsius other things (such as pressure) being equal. The temperature at which energy is available is a good measure of its quality - the higher the temperature of the energy, the more mechanical work we can theoretically get out of it. This maximum theoretical efficiency, called the Carnot efficiency, allows us to compare how well any particular real-world energy-using system is performing relative to the maximum theoretical performance. In 1824, a French physicist, Nicholas Léonard Sadi Carnot, described the most efficient (ideal) engine for converting heat to mechanical work. By contrast, the second law of thermodynamics allows us to know how well an energy system performs in terms of the quality of the energy. The first law of thermodynamics states that energy is conserved even when its form is changed, as for instance from mechanical energy to heat. Application of the second law of thermodynamics helps explain the various ways in which engines transform heat into mechanical work, as for instance in the gasoline engine of a car or in a steam turbine.Įfficiency measures based on the second law of thermodynamics take into account the quality of energy - unlike efficiencies based on the first law of thermodynamics which take into account only the amount of energy. The second law of thermodynamics states that you can move heat from a hotter place to a colder place without doing work, but that you need to work to move heat from a colder place to a hotter place.
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