Conversion of Thermal Energy into Work

  • #1
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. Homework Statement ?

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

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?
 

Answers and Replies

  • #2
Bystander
Science Advisor
Homework Helper
Gold Member
5,272
1,325
why the above argument is wrong?
You'll have to give us some idea what has been covered for you in "IB."
 
  • #3
21,493
4,875
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
 

Related Threads on Conversion of Thermal Energy into Work

  • Last Post
Replies
7
Views
1K
  • Last Post
Replies
2
Views
8K
Replies
1
Views
2K
  • Last Post
Replies
1
Views
15K
  • Last Post
Replies
2
Views
6K
Replies
2
Views
760
Replies
2
Views
24K
Replies
3
Views
1K
  • Last Post
Replies
0
Views
354
M
  • Last Post
Replies
2
Views
1K
Top