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sign of q

Posted: Thu Feb 01, 2018 4:19 pm
by Hannah Krusenoski 2L
When calculating entropy how do you know when qrev is negative in the equation?

Re: sign of q

Posted: Thu Feb 01, 2018 5:43 pm
by Sean Monji 2B
Qrev is the heat being transferred into or out of the system. If heat is leaving the system, the q would be negative. If heat enters the system, then q is positive. This makes sense since if there is more energy in the system, you can expect there to be an increase in entropy.

Re: sign of q

Posted: Thu Feb 01, 2018 8:02 pm
by Varsha Swamy 2J
It might help to think of it using the familiar concept q(system)=-q(surroundings). A system's surroundings gain the heat that a system loses. Because a gain in heat is an increase or positive value, the system must have the opposite sign when it loses heat. So if heat is leaving a system, q will be negative.

Re: sign of q

Posted: Fri Feb 02, 2018 1:50 pm
by Charles Ang 1E
I also have trouble with the sign of q. I found that it helps me to try to think whether the q applies to the system or surrounding and determine what is losing and what is gaining.

Re: sign of q

Posted: Sat Feb 03, 2018 3:32 pm
by Ramya Lakkaraju 1B
The system's q sign and the surrounding's q sign will always be opposites since whatever the system is losing is what the surroundings gain.

Re: sign of q

Posted: Sun Feb 04, 2018 8:09 pm
by Sabah Islam 1G
When qrev is negative, it means that the system is losing heat, so the surroundings will gain heat and vice versa if qrev is positive. You can think of it in the context of exothermic and endothermic: if a system is endothermic (taking in heat), then qrev will be positive and if a system is exothermic (releasing heat), then qrev will be negative.

Re: sign of q

Posted: Sun Feb 04, 2018 8:16 pm
by Abigail Urbina 1K
The sign of delta S depends on the direction of heat flow. Delta S is simply qrev divided by T. qrev is negative whenever the system is losing heat. From that information, you then know that the surroundings gain heat. If the system is gaining heat, then qrev is positive.

Re: sign of q

Posted: Sun Feb 04, 2018 9:30 pm
by Arshpreet Sandhu 1B
q of system= -q of surroundings