Three hundred joules of heat are added to a system. The system uses 140 joules to do work. By how much did the internal energy of the system change?

By the first law of thermodynamics, the change in internal energy is the heat added minus the work done by the system: ΔU = Q − W. Substituting the values gives ΔU = 300 J − 140 J = 160 J. The internal energy increased by 160 J, so the correct choice is 160 J.

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