Ch20; Heat and the First Law of Thermodynamics
The First Law
This is just an expansion of the ideas of Energy Conservation or the Work-Energy Theorem.
U = internal energy of the system
Q = heat transferred into the system
W = work done by the system on its surroundings
U = Uf - Ui
U = Q - W
That is, if energy flows into the system in the form of heat Q, some of that can be used to do work W by the system on its surroundings and the rest will go to increasing the internal energy of the systemU;
Q = W + U
We could also write this First Law of Thermodynamics in the form of
dU = dQ - dW
for very small changes.
Remember our sign conventions:Positive heat Q means heat flows into the system while negative Q means heat flows out of the system. Positive work W means the system does work on its surroundings (like an expanding piston) while negative W means the surroundings does work on the system (like a piston compressing the system).
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