Find the change in internal energy of the system when a system absorbs 2 kilocalorie of heat and at the same time does 500 joules of work.
Correct answer: C. 7900 J
- A. 8200 J
- B. 5600 J
- C. 7900 J
- D. 6400 J
Explanation
First, convert the heat absorbed to Joules: Q = 2 kcal × 4200 J/kcal = 8400 J (using 1 cal ≈ 4.2 J). According to the first law, ΔU = Q - W. Here Q = +8400 J and W = +500 J. Therefore, ΔU = 8400 J - 500 J = 7900 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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