Conflicting Sign Conventions: Physics vs. Chemistry - Work

[ilovemcat]

From a Chemistry Perspective: deltaE = q + W (W = -PdeltaV)
if a system does work on the surroundings, Work is negative
if the surroundings does work on the system, Work is postive

However, the opposite is true for Physics: deltaE = q - W (W = PdeltaV)
if a system does work on the surroundings, Work is positive
if the surroundings does work on the system, Work is negative

My question is: What convention would we use for the MCAT? Sure it wouldn't matter if we're considering change in Internal Energy - but what if we were asked about work. What sign convention do we use?

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[IntelInside]

From a Chemistry Perspective: deltaE = q + W (W = -PdeltaV)
if a system does work on the surroundings, Work is negative
if the surroundings does work on the system, Work is postive

However, the opposite is true for Physics: deltaE = q - W (W = PdeltaV)
if a system does work on the surroundings, Work is positive
if the surroundings does work on the system, Work is negative

My question is: What convention would we use for the MCAT? Sure it wouldn't matter if we're considering change in Internal Energy - but what if we were asked about work. What sign convention do we use?

The work that an object does is always positive but we use the sign conventions to keep track of internal energy change i.e the gain or loss of energy.

 

[Rabolisk]

The question will indicate which sign to use by asking for the work done BY the Sustem or the work done ON the system. Conversely they can also ask how much work is done by the SURROUNDINGS or the work done on the SURROUNDINGS.

 

[UIUCstudent]

The work that an object does is always positive but we use the sign conventions to keep track of internal energy change i.e the gain or loss of energy.

I use the same thinking. Let's say what's the work done on the ground as a book slides against it. It's positive since the ground is gaining energy in the form of heat.

So for the piston, as volume increases, we would say it's losing energy to the surrounding so that's negative work.

I also keep the caveat that Rabolisk mentioned in the back of my head.

 

[ponyo]

Not to dig up an old thread, but AAMC says U = q + w. Does that mean we should go with the acquisitive view in general (work done on gas being positive)?

I'm pretty sure I understand the concepts but I don't want wrong formula impressions to confuse me on the MCAT. Having to work formula out from basic concepts is a real btch.

 

[sps27]

Bumping an ole thread. Ran into this myself and had some confusion which sign convention to use.

 

[FeinMS]

You will get the same answer no matter which formula you use...

 

[Blueprint MCAT Tutor]

The question will indicate which sign to use by asking for the work done BY the Sustem or the work done ON the system. Conversely they can also ask how much work is done by the SURROUNDINGS or the work done on the SURROUNDINGS.

This is the credited response. Remember, the MCAT is a reading test, not a science test. Read the question carefully and they'll make it clear what they're looking for.