work done by or on system

[TotalDomination]

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confused which is positive and which is negative. can someone clarify?

 

[mcgst9]

confused which is positive and which is negative. can someone clarify?

I believe work done by the system is subtracting energy from the system itself and thus is negative.

Work done on the system is putting energy into the system, and thus it is positive.

I think that's how it is. 😛

 

[TotalDomination]

I believe work done by the system is subtracting energy from the system itself and thus is negative.

Work done on the system is putting energy into the system, and thus it is positive.

I think that's how it is. 😛

So work done by system decreases internal energy so is negative. work done on system is increasing internal energy so positive? I think i just restated what you said but just to make sure.

 

[tchoudha]

u=q-w

if the system is expanding, w is +
if the system is compressed, w is -
if there is no heat flow, then q=0. Thus u=+/-w
eg if it's an isothermal compression, work is done on the system, work is neg. thus the system is gaining k.e
u=q-w
u=0-(-w), u=w, thus k.e. is gained

if it's an isothermal expansion, work is done by the system, work is positive. Thus the system is losing k.e.
u=q-w
u=0-(+w), u=-w, thus k.e is lost

 

[Mindbuffer]

It depends if you're using the equation ΔU = Q - W or ΔU = Q + W.

I think Kaplan says work done by the system is positive, whereas Princeton says it's negative. I'm sure AAMC would give the equation if they were to test this.

 

[brood910]

It depends if you're using the equation ΔU = Q - W or ΔU = Q + W.

I think Kaplan says work done by the system is positive, whereas Princeton says it's negative. I'm sure AAMC would give the equation if they were to test this.

Then Kaplan is wrong.
It is negative.

W = -PV

Work by system = expansion = change in V is +. So W is negative.

This is why you gotta love TBR.

 

[oxelhs]

Then Kaplan is wrong.
It is negative.

W = -PV

Work by system = expansion = change in V is +. So W is negative.

This is why you gotta love TBR.

Yes. Just remember work ON system = + and thus work done BY system = -

 

[Mindbuffer]

Trust me, I've been over this multiple times in the past. No one is right or wrong, it simply depends on what form of the equation you use. This is why the AAMC's MCAT will not test this without giving the equation.

 

[brood910]

Trust me, I've been over this multiple times in the past. No one is right or wrong, it simply depends on what form of the equation you use. This is why the AAMC's MCAT will not test this without giving the equation.

Not really.
Even in my undergraduate class, I learned that it is negative.

 

[Mindbuffer]

Not really.
Even in my undergraduate class, I learned that it is negative.

That's great, but that doesn't mean that Kaplan is wrong. They just use a different convention as I mentioned in my previous post. If you notice something in AAMC material where they expect you to determine the sign of the work without giving the equation, then maybe you can make a point.

 

[brood910]

It does mean Kaplan is wrong.
With your logic, equations for potential energy should be able to be used without a negative sign as well then. This will mess up the whole concepts.

 

[Mindbuffer]

It does mean Kaplan is wrong.
With your logic, equations for potential energy should be able to be used without a negative sign as well then. This will mess up the whole concepts.

No, it's not my logic. What I'm saying applies only to this concept. Some textbooks teach U = Q + W whereas others teach U = Q - W. Neither is right or wrong, but they determine the sign of the work.

 

[brood910]

No, it's not my logic. What I'm saying applies only to this concept. Some textbooks teach U = Q + W whereas others teach U = Q - W. Neither is right or wrong, but they determine the sign of the work.

If they use U = Q + W, then they use W = PV, not W = -PV. Problem solved.

 

[SN2ed]

Thread closed. These types of questions belong in the MCAT Study Q&A forum.