Answer to Question #185588 in Molecular Physics | Thermodynamics for Max

Question

Explain entropy

Answer 1

When two systems of different temperatures are in thermal contact, heat flows out of the hotter system and into the colder system. There is no change in the total energy of the two systems; energy just flows out of one and into the other. Why then does heat flow in one direction but not in the other? Heat flow into a system not only increases the system’s internal energy, it also increases the disorder of the system. Heat flow out of a system decreases not only its internal energy but also its disorder.

The entropy of a system (symbol S ) is a quantitative measure of its disorder. Entropy is a state variable (like U, P, V, and T ): a system in equilibrium has a unique entropy that does not depend on the past history of the system.

If an amount of heat Q flows into a system at constant absolute temperature T, the entropy change of the system is

$ΔS = \frac{Q}{T}$