work done by system or on system


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JacobHershenhouse3G
Posts: 32
Joined: Thu Jan 10, 2019 12:17 am

work done by system or on system

Postby JacobHershenhouse3G » Tue Feb 05, 2019 8:39 pm

I know that deltaU= q + w but how do you differentiate if work and temperature is applied to or by the system from the surroundings? I seem to keep messing up signs and would like some clarification.

ChathuriGunasekera1D
Posts: 78
Joined: Fri Sep 28, 2018 12:17 am

Re: work done by system or on system

Postby ChathuriGunasekera1D » Tue Feb 05, 2019 9:09 pm

Hi! Here is a basic set of guidelines
q is positive if system absorbs energy
q is negative if system releases energy
w is positive if work is being done on the system
w is negative if work is being done by the system

An example is going from solid to liquid; it requires heat to break the IMFs, so the q is positive because energy is absorbed. Because work is being done by the system, w is negative.

JiangJC Dis2K
Posts: 72
Joined: Fri Oct 12, 2018 12:16 am

Re: work done by system or on system

Postby JiangJC Dis2K » Tue Feb 05, 2019 9:09 pm

if q for the system is positive, then it absorbs heat. if q is negative, it releases heat and heat is therefore lost to the surroundings. if w is positive for the system, work is done on the system by the surroundings. if w is negative, work is done by the system on the surroundings. hope this helps!

Vicky Lu 1L
Posts: 60
Joined: Fri Sep 28, 2018 12:18 am

Re: work done by system or on system

Postby Vicky Lu 1L » Tue Feb 05, 2019 10:58 pm

q (heat) is positive when its has key words such as absorbed/gain energy and is negative when it released/emitted/loses energy. w (work) is positive when work is done on the system and negative when the system does or is doing the work.


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