Physics Ch4 Independent Question 5

vgp1993
Posts: 5
Joined: Wed Dec 20, 2017 12:14 pm

Physics Ch4 Independent Question 5

Postby vgp1993 » Sat Jan 13, 2018 2:39 pm

This question starts by giving you the work done by the gas on the piston, so I was setting up the equation deltaU= Q - W in my head and was expecting to answer it as a +25J change in energy for the gas. However, When I got to the last three words of the question and saw that it was talking about the energy change in the "gas-piston system" (not just the gas), I decided to choose 0J because now the work is being exerted on something that we are considering part of the system by the system.

Can someone tell me why this is incorrect?
NS_Tutor_Andrew
Site Admin
Posts: 286
Joined: Mon May 23, 2016 1:47 pm

Re: Physics Ch4 Independent Question 5

Postby NS_Tutor_Andrew » Sun Jan 14, 2018 8:30 pm

Hi vgp1993,

Excellent question! If I understand your reasoning correctly, that would result in an answer of 70 J, not 0 J, though, because you would still have to account for the 70 J of energy that the question describes as coming from out of the system. From the point of view of pure question-solving strategy, the fact that this is not an answer choice indicates that you must think about it differently, and numerically, the answer choices suggest that you're going to be playing around with different combinations of adding/subtracting 70 J and 45 J.

To respond to your substantive point, though, I don't think the question gains anything by asking about the "gas-piston system" rather than just the gas, so we will revise the question to avoid the potential distractor issue of whether the work performed on the piston results in no energy change within the system.

Hope this clarifies thing, & thanks again for the question!
Andrew D.
Content Manager, Next Step Test Prep.

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