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### U and its relations to work

Posted: Fri Feb 01, 2019 8:50 am
What does the variable U stand for and what is its relation to work?

### Re: U and its relations to work

Posted: Fri Feb 01, 2019 8:55 am
U, stands for the internal energy of a system which can be altered by heat flow and work.
If heat flows into the system; work is done on the system, and its internal energy increases, ΔU > 0.
If heat flows out of the system, work is done by the system, and its internal energy decreases, ΔU < 0

Hope this helps!

### Re: U and its relations to work

Posted: Fri Feb 01, 2019 11:21 am
Additionally in a closed system, internal energy relates to work in this equation:

delta U = q + w

### Re: U and its relations to work

Posted: Fri Feb 01, 2019 1:32 pm
U represents the internal energy of a system. Its relation to work can be seen in the first law of thermodynamics: change in U = q(heat exchanged) + w(work done on or by the system).

### Re: U and its relations to work

Posted: Fri Feb 01, 2019 1:35 pm
The equation that relates U to work is delta U= q+w, U represents the change in internal energy.

### Re: U and its relations to work

Posted: Fri Feb 01, 2019 1:43 pm
U is a state property and is the internal energy of a system. Changes in internal energy are a function of work. The energy of a closed system can be changed by heating/cooling or compression/expansion. $\Delta$U=q + w. If $\Delta$ V is zero (constant volume), delta U= $q_{v}$. If $\Delta$P=0 (constant pressure), then $\Delta$U=$\Delta$H-P$\Delta$V.

### Re: U and its relations to work

Posted: Sat Feb 02, 2019 12:35 pm
What is the difference between q and delta H?

### Re: U and its relations to work

Posted: Sat Feb 02, 2019 7:54 pm
Additionally, when do you know whether to use q or delta H?

### Re: U and its relations to work

Posted: Sat Feb 02, 2019 8:20 pm
Delta H is enthalpy while q is heat flow.