Explain in which case do we use ∆U = Q + W - eq(1) Or ∆U = Q - W – eq?
2. According to the First Law of Thermodynamics, ∆U = Q + W - eq(1) Or ∆U = Q - W – eq(2).
Where:
∆U = internal energy of the system
Q = Amount of heat
And W = work done
Explain in which case do we use eq (1) and where do we prefer eq (2)?
Answers & Comments
Verified answer
Why would you use either of these two equations?
Typically we're looking for the total thermal energy Q = U +/- W; where + W means work is done on the system so energy is added to the internal energy U and - W means work is done by the system so energy is used up from U.
In this format, it should be clear to you now. Q - W = U is when work energy is added to the system and Q + W = U is when its done by the system and lost. Bottom line, put the W term on the other side of the = sign and you'll see which way the work is going...added to or subtracted from U