# Conversion of Thermal Energy into Work

1. Sep 25, 2015

### 3432401GSPT

This question appeared in my IB Physics HL mock exam and I am stumped by the answer on the mark sheet. It really should be a trivial question but it's open to interpretation which is why I find it unreasonable. It was in Paper 1 from November 2014,

1. The problem statement, all variables and given/known data?

Which of the following is true when thermal energy is converted into work in a single process and a cyclical process:
Single process: / Cyclical process:
A: complete conversion of thermal energy into work can occur / energy must be transferred from system
B: complete conversion of thermal energy into work can never occur / energy must be transferred from system
C: complete conversion of thermal energy into work can occur / energy need not be transferred from system
D: complete conversion of thermal energy into work can never occur / energy need not be transferred from system

3. The attempt at a solution

I was totally happy eliminating answers C and D which left A and B. I answered B. The answer on the mark sheet was A.

The only way I can rationalize the answer comes from the possibility of a slow isothermal expansion of an ideal gas with a piston moving into a vacuum (so with a heat reservoir so the gas in the piston is at thermal equilibrium). However, an ideal gas is exactly that. If they had specified an ideal gas in the question I would have less trouble agreeing with this answer, however, I think the reality is, is that I am pulling in some heat from the surroundings to overcome the attraction between molecules, even if such an energy transfer is very minute. So while according to the ideal gas model A is correct, what is true is B?

Would anyone be able to clarify as to why the above argument is wrong?

2. Sep 25, 2015

### Bystander

You'll have to give us some idea what has been covered for you in "IB."

3. Sep 26, 2015

### Staff: Mentor

Even for a substance that is not an ideal gas, so that the internal energy is a function of both temperature and pressure (or specific volume), there must be a way of getting from an initial state to the final state having the same internal energy, but not necessarily the same temperature and pressure.

Chet