The second law defines a quantity known as entropy, which has units of energy per degree or energy divided by temperature, and it says that this quantity can only increase or stay the same over time. It is usually stated for an isolated system for the same reason that the first law is usually stated for an isolated system: Just as the energy of an open system can change by coming and going from the surroundings, the entropy of an open system can decrease as long as more entropy is given off to the surroundings and the net change in entropy over time is positive. That’s basically the trick to what are called “dissipative systems.” They are becoming more organized but “paying for” that organization by mixing up their surroundings more than they are organizing themselves. I like to avoid these details, however, by saying instead that the second law says that any irreversible process increases the entropy of the universe. The microscopic, idealized processes of individual molecules or individual waves in mechanics are reversible and do not produce entropy. Any real, macroscopic process, however, is irreversible and does produce entropy. So let me try and explain what it means to “produce entropy.”
The thing to do now is leverage the fact that we understand, or maybe its better to say simply that we believe, that energy can be “neither created nor destroyed” and ask, given this fact, what do we mean when we say that we “use” energy? What is the actual meaning behind the phrases “We obtain energy from food” and the phrase “The energy for life comes from the sun?” Let’s start by looking at the earth as a “closed” thermodynamic system.