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Sign Conventions

Posted: Thu Jan 12, 2017 9:05 pm
by Katrina_Domingo_3G
Hello,

For internal energy and heat, what sign conventions are we using? (example: what is the sign of delta U when the system does work)
If anyone has a list that could help... :) Thank you!!

Re: Sign Conventions

Posted: Thu Jan 12, 2017 10:35 pm
by E_Villavicencio 2N
Hi Katrina! When the system does work the sign will be negative, because it is losing internal energy when it does that work. When work is being done on the system, the sign will be positive because the system is gaining energy.

For example, if a system expands, it will be doing work because it is pushing against an opposing force (except for expansion in a vacuum, where there's no opposing force and w=0), and therefore losing energy (negative sign). If the system is compressed, there is something exerting work over the system and transmitting energy to it, therefore it gains energy (positive sign).