A thermodynamic system undergoes a process in which its internal energy decreases by 300 J. If at the same time, 120 J of work is done on the system, find the heat lost by the system.
Correct answer: A. -420 J
- A. -420 J
- B. 420 J
- C. 80 J
- D. -80 J
Explanation
According to the First Law of Thermodynamics:Q = ΔU + WAs in the question, the internal energy is decreased by 300J, so ΔU will be -300J. And in physics, work done on the system is taken as negative, so W = -120J.Substituting the values, we get:Q = -300 + (-120) = -300 - 120= -420J.Therefore, the heat lost by the system is -420 J.
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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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