Explain the Second Law?

[Ethan Liang 1A]

In class we were told that the second law of thermodynamics is about how entropy changes whenever heat goes in and out of a system while T is constant. In high school, I learned that the second law states that the entropy of the universe as an isolated system will increase over time. Where does the temperature being constant come into that explanation?

[Nathan Chu 2G]

Hello,

You are correct in saying that the Second Law of Thermodynamics implies that systems spontaneously evolves to states of higher entropy. This is given by the equation, delta S = q/T, where the temperature remains constant. This equation proves that so long as energy is being provided (+q), delta S (entropy) is positive. The higher the entropy, the more disordered the system is. Temperature is held constant in this case because we are assuming that the substance is undergoing a phase change.

Hope this helps! Cheers.