<P> The second law of thermodynamics states that the total entropy can never decrease over time for an isolated system, meaning - a system which neither energy nor matter can enter or leave . The total entropy can remain constant in ideal cases where the system is in a steady state (equilibrium), or undergoing a reversible process . In all other real cases, the total entropy always increases and the process is irreversible . The increase in entropy accounts for the irreversibility of natural processes, and the asymmetry between future and past . </P> <P> Historically, the second law was an empirical finding that was accepted as an axiom of thermodynamic theory . Statistical thermodynamics, classical or quantum, explains the microscopic origin of the law . </P> <P> The second law has been expressed in many ways . Its first formulation is credited to the French scientist Sadi Carnot, who in 1824 showed that there is an upper limit to the efficiency of conversion of heat to work, in a heat engine . </P> <P> The first law of thermodynamics provides the basic definition of internal energy, associated with all thermodynamic systems, and states the rule of conservation of energy . The second law is concerned with the direction of natural processes . It asserts that a natural process runs only in one sense, and is not reversible . For example, heat always flows spontaneously from hotter to colder bodies, and never the reverse, unless external work is performed on the system . Its modern definition is in terms of entropy . </P>

When was the second law of thermodynamics discovered