Given below are the four cases in which certain heat transfer is taking place :
- 1. Ice is melting in a glass full of water
- 2. Water is boiling in an open container
- 3. A metal rod is heated in a furnace
- 4. A cup of coffee is allowed to cool on a table
In which of the above cases, the Newton’s Law of Cooling is applicable?
1 only
4 only
1 and 4 only
1, 2 and 3
Answer is Wrong!
Answer is Right!
This question was previously asked in
UPSC NDA-2 – 2024
1. Ice melting: Involves a phase change at a constant temperature (0°C). The rate of heat absorption is constant if the surroundings are at a constant temperature, but the process is melting, not cooling according to the temperature-difference proportionality of Newton’s law.
2. Water boiling: Involves a phase change at a constant temperature (100°C at atmospheric pressure). This is a process of heating (adding heat to cause vaporization), not cooling.
3. Metal rod heated in a furnace: This is a process of heating the rod by transferring heat *from* the furnace *to* the rod. Newton’s Law of Cooling describes heat *loss* (cooling).
4. Cup of coffee cooling on a table: The hot coffee loses heat to the cooler surroundings (air and table) via convection and radiation, causing its temperature to decrease. This scenario directly fits the conditions and description of Newton’s Law of Cooling, especially as the temperature difference between the coffee and the surroundings decreases over time.