If a thermodynamic system is taken along a path A along which -25.4 J flows into

Question

If a thermodynamic system is taken along a path A along which -25.4 J flows into the system and 76.4 J of work is done on the system, what is the change in internal energy, ?U? If the same system is taken along a second path B for which the endpoints are the same so that ?U is the same, but 76.2 J of heat Q flows into the system, what is the work W done along that path? (?U only depends on the endpoints of the path, not the whole path. Q and W depend upon the whole path.) If further this same system is taken along a third path C for which the endpoints are the same so that ?U is the same, but -35.5 J of work W is done on the system, how much heat, Q, flows into the system?

Explanation / Answer

A) heat flowing thru the system, Q = -25.4J

Work is done on the system, therefore, W = -76.4J

Using 1st Law of thermodynamics

U = Q - W = -25.4 -(-76.4) = 51J

B) U = 51J , Q = 76.2J

U = Q - W

W = Q - u = 76.2 - 51

W = 25.2J

C)U = 51J , W = -35.5J

U = Q - W

Q = U + W = 51 + (-35.5)

Q = 15.5J

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