Q. In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following?- Area of the hole in the lid
- Temperature of the flame
- Weight of the lid
Select the correct answer using the code given below. (UPSC Prelims 2021)
Answer:
1 and 3 only
Notes: The correct answer is
[C] 1 and 3 only. The fundamental principle of a pressure cooker is that it increases the boiling point of water by increasing the internal pressure.
- Area of the hole in the lid (Statement 1 – Correct): The pressure inside the cooker is defined as Force / Area. The hole (steam vent) is where the internal steam exerts upward force against the weight (whistle). If the area of this hole changes, the pressure required to lift the weight changes, thereby altering the internal boiling temperature.
- Weight of the lid/whistle (Statement 3 – Correct): The weight sitting on the vent determines the "release pressure." A heavier weight requires higher internal steam pressure to be lifted. According to the Gay-Lussac's law (P \propto T), higher pressure leads to a higher boiling point of water, allowing food to cook at temperatures well above 100°C.
- Temperature of the flame (Statement 2 – Incorrect): The flame temperature affects the rate at which water reaches its boiling point (how fast the cooker builds pressure), but it does not determine the final cooking temperature. Once the operating pressure is reached and the weight lifts to release steam, the internal temperature remains constant regardless of whether the flame is medium or high.
Physics of the Pressure Cooker:- Boiling Point Elevation: In an open vessel, water boils at 100°C at sea level. In a pressure cooker, the pressure can rise to nearly 2 bar, raising the boiling point to approximately 120°C.
- Heat Transfer: The higher temperature significantly reduces cooking time because the rate of heat transfer to the food is much greater at 120°C than at 100°C.