# B Entropy unavailable to do work

1. Oct 20, 2016

I was helping my cousin with her Physics II class and they just started discussing entropy in the Thermo section of the class and her teacher gave the "measure of disorder in a system" definition which I personally hate because even though it might be technically accurate, it doesn't really explain the concept. So I was trying to think of a better way to explain it to her, and I came across a definition on a website that said entropy was a measure of the energy in a system that's unavailable to do work, which didn't make sense to me. In an internal combustion engine, combustion of the fuel increases the entropy in the system and the combustion drives the cycle. Also, the integral of the T-s diagram is the heat transferred to the system, so you can't have net work without entropy, right?

2. Oct 20, 2016

### Grinkle

I think it refers to how close to uniform temp a closed system is at. If a closed system is a uniform temperature, there is energy present but none of it can be used to do work, and in such a system I think entropy is at a maximum possible value for that system.

3. Oct 20, 2016

### Staff: Mentor

The engine is based on a cycle, so the working substance of the engine must return to its initial state, hence be "entropy neutral." All the entropy created by the combustion (or coming from the hot reservoir) has to be dumped as heat energy in the cold reservoir. So yes, in a sense entropy corresponds to energy that can't perform work.