Ch20; Heat and the First Law of Thermodynamics

The First Law

of Thermodynamics

  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).

Work and Heat


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(c) Doug Davis, 2002; all rights reserved