UPSC Prelims 2001 Q102: Science & Technology – Physics
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is NOT a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (b) Both A and R are individually true but R is NOT a correct explanation of A
Explanation
Assertion-Reason: copper feels hotter than glass at the same temperature
| Statement | Verdict |
|---|---|
| A. Copper and glass at the same temperature — copper feels hotter | True |
| R. Copper is denser than glass | True — copper about 8.9 g/cm³, glass about 2.5 g/cm³ |
| Real reason for A | Copper has much higher thermal conductivity (about 400 W/m·K vs about 1 for glass), so it passes heat to the hand faster |
| Link | Density does not decide how hot a body feels — R does not explain A |
Both statements are true, but density is not why copper feels hotter. The hand feels the rate at which heat flows into it. Copper is an excellent conductor of heat, so it delivers heat much faster than glass at the same temperature. So R is true but not the explanation. (The bank stored (c); copper is clearly denser than glass, so R cannot be false.)
Answer: (b) Both A and R are individually true but R is NOT a correct explanation of A.
Why each option is right or wrong
- (a) Incorrect. Density does not explain why copper feels hotter; conductivity does.
- (b) CORRECT. Both are true, but thermal conductivity, not density, explains A.
- (c) Incorrect. R is true; copper is far denser than glass.
- (d) Incorrect. A is true; copper does feel hotter.
Key point
The same effect makes metal feel colder than wood on a cold morning: metal draws heat from the hand faster.
Remember
Copper feels hotter because of high thermal conductivity, not density.
Common mistakes
- Marking R false without checking the densities.
- Thinking density controls how hot a body feels.
- Forgetting thermal conductivity as the cause.
Exam tip
In A-R questions, test R on its own first, then ask if it really causes A.
Sources
- NCERT Class XI — Physics Part II, chapter 'Thermal Properties of Matter' (conduction)
- NCERT Class VII — Science, chapter 'Heat'