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03 March 2011

Get the vocabulary right in the first law of thermodynamics!

Q and U from a They Might Be
Giants wiki
W represents work done on a gas. W cannot increase or decrease. W can be positive or negative; negative W means work is done BY a gas.

Q represents heat added to a gas. Q cannot increase or decrease. Q can be positive or negative; negative Q means heat is REMOVED from a gas.

U represents internal energy of a gas. U can increase or decrease.

ΔU represents the change in internal energy. ΔU cannot increase or decrease.  ΔU can be positive or negative; negative ΔU means the internal energy DECREASES.
This was all posted to a class folder yesterday, after I graded a problem about a PV diagram and the first law of thermodynamics.  The 4 students who used illegal phrasology* such as "W decreases" lost credit.  I thought it was worth a classwide reminder with our exam coming up today...

* penalty: 5 yards and loss of down

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