UPSC Prelims 2021 Q68: Science & Technology – Physics
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (c) 1 and 3 only
Explanation
Pressure Cooker: What Determines Cooking Temperature?
| Factor | Affects Cooking Temperature? | Mechanism |
|---|---|---|
| 1. Area of hole in the lid | YES | The hole (steam vent) size determines how much steam escapes. A smaller hole retains more steam, raising internal pressure and boiling point. |
| 2. Temperature of the flame | NO (indirectly affects speed, not final temperature) | Flame determines how quickly water boils but the max cooking temperature is set by pressure equilibrium, not flame. |
| 3. Weight of the lid (whistle weight) | YES | The weight of the whistle/pressure regulator controls the pressure at which steam escapes. Heavier weight = higher pressure = higher boiling point = higher cooking temperature. |
Answer: (c) 1 and 3.
Cooking temperature in a pressure cooker depends on internal pressure. At 1 atm extra (2 atm absolute), water boils at ~121°C (vs 100°C at 1 atm). Both the hole area and the whistle weight determine the pressure retained.
Why each option is right or wrong
- (a) Incorrect. Flame temperature (2) affects heating rate, not the equilibrium cooking temperature.
- (b) Incorrect. Flame temperature does not set the final cooking temperature.
- (c) CORRECT. Cooking temperature depends on internal pressure, which is controlled by (1) the area of the hole in the lid (steam vent) and (3) the weight of the lid/whistle. Both regulate pressure equilibrium. Flame only affects heating speed.
- (d) Incorrect. Flame temperature does not control cooking temperature - only the pressure equilibrium does.
Key point
Pressure cooker temperature = f(pressure). Pressure is set by (i) hole/vent area (smaller = higher pressure) and (iii) weight of whistle (heavier = higher pressure). At 2 atm, water boils at 121C vs 100C at 1 atm. Flame affects speed, not final temperature.
Remember
In a pressure cooker, the cooking temperature depends on the internal pressure, which is regulated by (1) the area of the hole in the lid and (3) the weight of the lid/whistle. Flame temperature only affects heating speed, not the final cooking temperature. Correct answer: (c) 1 and 3 only.
Common mistakes
- Assuming higher flame = higher cooking temperature. Once boiling starts, extra heat only produces more steam (which escapes), not higher temperature.
- Not knowing water's boiling point rises ~1C per 27 mm of Hg extra pressure.
- Confusing pressure cooker principle (raising boiling point) with normal cooking (fixed at 100C atmospheric).
Exam tip
Pressure cooker physics: P up = Boiling point up = Cooking temperature up. At sea level water boils at 100C; at 2 atm it boils at 121C (used for autoclaving). Small vent + heavy whistle = higher pressure = faster cooking. Great for high altitudes where water boils at <100C.
Sources
- NCERT Class IX Science - Chapter on Matter in Our Surroundings
- NCERT Class XI Physics - Chapter on Thermodynamics