Sixty-three joules of heat are added to a closed system. The initial internal energy of the system is 58 J, and the final internal energy is 93 J. How much work is being done by the system?

The internal energy changes by 93 J − 58 J = 35 J. Using the first law, ΔU = Q − W, where W is work done by the system, so W = 63 J − 35 J = 28 J. The system does 28 J of work, which is choice D.

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