Question

The change in internal energy of a system which has absorbed 2 kcal of heat and done 500 J of work is:

• A. 7900 J
• B. 8900 J
• C. 6400 J
• D. 5400 J

Hint:

Always convert all energy units to Joules before performing calculations ($1 \text{ cal} \approx 4.18 \text{ J} \text{ or } 4.2 \text{ J}$).

Answer 1

The Correct Option is A

Step 1: Concept
First Law of Thermodynamics: $\Delta Q = \Delta U + \Delta W$.
Step 2: Analysis
$\Delta Q = 2 \text{ kcal} = 2000 \times 4.2 \text{ J} = 8400 \text{ J}$. $\Delta W = 500 \text{ J}$. $\Delta U = \Delta Q - \Delta W = 8400 - 500$.
Step 3: Conclusion
$\Delta U = 7900 \text{ J}$.
Final Answer: (A)