Q and -Q

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Andrew Uesugi 3I
Posts: 38
Joined: Wed Sep 21, 2016 2:56 pm

Q and -Q

Postby Andrew Uesugi 3I » Fri Jan 27, 2017 6:16 pm

Hey, I just want to confirm something.

Whenever I see this: q=-q, does -q mean that there is heat gain or loss? I get kind of confused with this.

stephanieyang_3F
Posts: 62
Joined: Wed Sep 21, 2016 2:55 pm

Re: Q and -Q

Postby stephanieyang_3F » Fri Jan 27, 2017 8:06 pm

All this means is that the heat is being transferred. -q would mean that in respect to that point (whether it is sys/surrounding or wherever it's coming from) it would have lost heat and whatever is gaining that heat would have a positive q. Since energy is always conserved, the amount of heat lost would (-) = the amount of heat gained (+). You typically use this for questions that involve an equilibrium in temperature: you put heated metal into cold water and what's the final temperature? or qrxn = -qcal for constant volume bomb calorimeters. Hope this makes sense.

Annie Zhang_ 3C
Posts: 12
Joined: Wed Sep 21, 2016 2:59 pm

Re: Q and -Q

Postby Annie Zhang_ 3C » Sat Jan 28, 2017 3:17 pm

Hi,

Just to continue off the other response, this is basically the first law of thermodynamics. It states that "the internal energy of an isolated system is constant"; the universe can be considered an isolated system. Thus, we can say that (as Stephanie said) energy is always conserved, so any time that energy is lost, it is gained somewhere else (maybe in another form).


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