Using the first law of thermodynamics, ΔU = Q − W. If a system absorbs 120 J of heat and does 40 J of work, what is ΔU?

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Multiple Choice

Using the first law of thermodynamics, ΔU = Q − W. If a system absorbs 120 J of heat and does 40 J of work, what is ΔU?

Explanation:
The first law, ΔU = Q − W, tells how the internal energy changes when heat is added and work is done. Heat coming into the system is positive, so Q = +120 J. The system does 40 J of work, which means W = +40 J. Plugging in, ΔU = 120 J − 40 J = 80 J. So the internal energy increases by 80 J. If the numbers were different, you’d balance heat and work the same way: for no work, ΔU would be 120 J; for 120 J of work, ΔU would be 0 J, and so on.

The first law, ΔU = Q − W, tells how the internal energy changes when heat is added and work is done. Heat coming into the system is positive, so Q = +120 J. The system does 40 J of work, which means W = +40 J. Plugging in, ΔU = 120 J − 40 J = 80 J. So the internal energy increases by 80 J. If the numbers were different, you’d balance heat and work the same way: for no work, ΔU would be 120 J; for 120 J of work, ΔU would be 0 J, and so on.

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