Question 33:
One calorie (cal) equals to how many joules (J)?
One calorie equals 4.18 joules, making option (B) correct for this physics question.
Question Solution
A calorie (cal) measures heat energy needed to raise 1 gram of water by 1°C, while the joule (J) is the SI unit for all energy forms. The exact conversion, set experimentally as the mechanical equivalent of heat, is 1 cal = 4.184 J, often rounded to 4.18 J in exams.
Option Analysis
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(A) 2.18: Too low; underestimates by half, not matching experiments like Joule’s paddle wheel tests.
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(B) 4.18: Correct; aligns with standard value (4.184 J rounded), used in physics and nutrition contexts.
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(C) 6.18: Overestimates; no basis in measurements, far from accepted factor.
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(D) 7.18: Highest and incorrect; exceeds real conversion by 70%+.
One calorie equals how many joules? This key physics conversion—1 cal = 4.18 J—matters for competitive exams like GATE Life Sciences and everyday energy calculations. Understanding it helps in biochemistry, nutrition, and thermodynamics problems.
Calorie to Joule Basics
A calorie is the heat to raise 1g water by 1°C; 1 joule is work from 1 newton over 1 meter. James Joule’s 1840s experiments fixed 1 cal = 4.184 J (often 4.18 J). Formula: J = cal × 4.18. Example: 5 cal = 20.9 J.
Why 4.18 J?
It’s empirical, from converting mechanical work to heat via specific heat (4.184 J/g°C for water). Variations exist (e.g., 4.186 J international), but 4.18 J suits most MCQs.
| Calories | Joules (×4.18) |
|---|---|
| 1 | 4.18 |
| 10 | 41.8 |
| 100 | 418 |
| 1000 (1 kcal) | 4180 |
Exam Tips
For GATE-style questions, pick 4.18 J over approximations. Avoid pitfalls like confusing cal with kcal (1 kcal = 4180 J). Practice: Convert 20 cal to J = 83.6 J.
