The Second law of thermodynamics makes an important statement about the heat balance in a thermodynamic system, such as the atmosphere. Like the First law, it also emphasizes that the total heat energy supplied to a system remains constant and goes partly to increase the internal energy of the system and partly to do work against external pressure. But there is a difference. The First law does not tell us how much of the given heat energy goes to increase the internal energy and how much to do the external work in any heat transfer process. The only stipulation is that the total of the two should remain constant. It was, however, known from early times that physical processes involving friction, turbulence, conduction, etc, dissipated a good part of the heat energy supplied to a thermodynamic system and only a partial conversion of heat into useful mechanical energy was possible by using even the best of heat engines. The manner in which heat was supplied to, or withdrawn from, a working substance made a lot of difference.
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