Thirty joules of heat flow into a system. The system in turn does 50 Joules of work. The internal energy of the system has:
Correct answer: D. Decreased by 20 joules
- A. Increased by 80 joules
- B. Decreased by 80 joules
- C. Increased by 20 joules
- D. Decreased by 20 joules
Explanation
Using the first law of thermodynamics, ΔU = Q - W. Here, heat flows in, so Q = +30 J, and the system does work, so W = +50 J. Therefore, ΔU = 30 J - 50 J = -20 J, indicating the internal energy has decreased by 20 joules.
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About First Law of Thermodynamics
The first law relates heat supplied, work done and the change in internal energy through energy conservation. Problems use sign conventions and apply the law to isothermal, adiabatic, isobaric and isochoric processes. Internal energy is a state function, while heat and work depend on the path followed.
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