Adiabatic Process Q

[Deepa100]

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Q: For a perfectly insulated system, what are the values of deltaE(internal) and q if w=+100J?
(a) deltaE(internal) =-100J;q=0
(b) deltaE(internal) =0;q=-100J
(c) deltaE(internal) =+100J;q=0
(d) deltaE(internal) =0;q=+100J

 

[physics junkie]

E = Q + W

In an adiabatic process no heat enters or leaves the system. This means Q = 0. You can still perform work in an adiabatic process.

100 = 0 + 100.

 

[Deepa100]

E = Q + W

In an adiabatic process no heat enters or leaves the system. This means Q = 0. You can still perform work in an adiabatic process.

100 = 0 + 100.

So, is the work done by the system or on the system? Is the answer A or C?

 

[physics junkie]

So, is the work done by the system or on the system? Is the answer A or C?

Sign designations are arbitrary. Work done by the system can be defined as positive or negative. You get to choose. However, once you choose work done by the system to be a certain sign then you absolutely have to be consistent.

In this case I'm assuming your book indicated what convention they were going to use(work by the system is positive or work by the system is negative).

I would assume that you just plug w=100 to get E = 0 + 100 so C is the answer. It would be unnecessarily tricky otherwise and wouldn't add a layer of depth to the problem as you are only dealing with one quantity that you have to assign a sign to.

 

[BerkReviewTeach]

Sign designations are arbitrary. Work done by the system can be defined as positive or negative.

Excellent point physics junkie! This is so important for test-takers to realize. In chemistry and physics, the sign convention is defined in an opposite way, because chemists are flask centric and physicists are concerned with what work we can get out of a system. If a system expands, then everyone agrees that work was done by the system on the surroundings. A physicist sees this as Positive work because it can be harnassed. However, the chemist sees it as Negative work because the molecules in the flask lost the energy. This is why a chemistry asigns a -# to an exothermic reaction while a physicist counts the heat that can be extracted from the system.

So, is the work done by the system or on the system? Is the answer A or C?

You'll need to take the lead from the question or passage as to how they are applying the sign convention if a question of this type arises. As the junkie has pointed out, it cannot be answered without first deciding on a convention.