Entropy is a measure of disorder in a system.

The entropy is: S = k ln (Number of available states) and has units J/K.

While any thermodynamic system an entropy value, it is simplest to determine the entropy change in a thermodynamic process.

Entropy change ΔS occurs whenever there is a transfer of heat.
ΔS = S_f - S_i = ∫
dQ

T

If heat transfer takes place at a fixed temperature, the entropy change is:
ΔS =
Q

T
(isothermal process)