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QPage 46: Q2: HC Verma: Concepts of Physics - Volume 2 Calorimetry Page 46: Q2 Solution: The calorie is defined as . Why not as to make the conversions…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often treat the calorie‑to‑joule factor as a convenient approximation and round 4.186 J to 4 J, forgetting that the calorie is defined by a precise thermodynamic process (raising 1 g of water by 1 °C). Because unit definitions are exact, rounding changes the energy value and violates the heat‑work equivalence; always use the defined conversion without truncation.

The calorie is defined as 1 cal=4.186 J1 \text{ cal} = 4.186 \text{ J}. Why not as 1 cal=4 J1 \text{ cal} = 4 \text{ J} to make the conversions easy?

Step-by-Step Verified Solution

**Answer:** One calorie is defined as the heat required to raise the temperature of 1 g of water from 14.5�C to 15.5�C. Exactly 4.186 J of work is required to do this. We cannot round off 4.186 J to 4 J as it would give a significant difference in work done required to raise the temperature and heat required to raise the temperature. Since work and heat are equivalent here, taking 1 calorie = 4 J would defy the validity of the equivalence of work and heat.[citation:1][citation:13]

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