## example 4A.1

$\Delta U=q+w$

Skyllar Kuppinger 1F
Posts: 52
Joined: Thu Jul 25, 2019 12:16 am

### example 4A.1

"in the internal combustion engine in an automobile, hot compressed gas expands against a piston, turning the driveshaft that moves the vehicle along the road. If you are investigating the performance of such an engine, you might need to assess the work that each stroke of the piston can achieve. Suppose a gas expands by 500. ml (0.500L) against the opposing force of 1.20 atm and no heat is exchanged with the surroundings during the expansion.

How much work is done in the expansion?

So I know that work= -PdeltaV. so in this case p is 1.20 and delta v is 0.5. This yields -0.600 Latm....I thought work was already in joules? when you calculate work by using work=force x distance, is this already in joules or is that also in some other unit which needs to be converted to joules?

Also, I know that work being done BY the system is represented as negative, but if the question (lets say on a test) specifically asks how much work did the system do, do I still need to include the negative? Cus saying "the system did 5 joules of work" is the same thing as saying "Work=-5 joules". The negative sign would be redundant.

MingdaH 3B
Posts: 133
Joined: Thu Jul 11, 2019 12:17 am

### Re: example 4A.1

The answer is already in joules when you plug it into the equation. Writing the answer either way works, as long as you make sure that it is clear what the direction of work is.

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