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  • A glass full of hot milk is poured on the table. It begins to cool gradually. Which of the following is correct? a) the rate of cooling is constant till milk attains the temperature of the surrounding

A glass full of hot milk is poured on the table. It begins to cool gradually. Which of the following is correct?

a) the rate of cooling is constant till milk attains the temperature of the surrounding

b) the temperature of milk falls off exponentially with time

c) while cooling, there is a flow of heat from milk to the surrounding as well as from surrounding to the milk but the net flow of heat is from milk to the surrounding and that is why it cools

d) all three phenomenon, conduction, convection, and radiation are responsible for the loss of heat from milk to the surroundings

Answers (1)

The answer is the option (b) The temperature of milk falls off exponentially with time, (c) While cooling, there is a flow of heat from milk to the surrounding as well as from surrounding to the milk but the net flow of heat is from milk to the surrounding and that is why it cools and (d) All three phenomenon, conduction, convection and radiation are responsible for the loss of heat from milk to the surroundings.

Explanation:

  1. Loss of heat is proportional to temperature difference with surrounding and body. As milk cools, rate of cooling decreases with time. Thus, opt (a) is incorrect.
  2. The heat of milk falls exponentially by Newton’s law of cooling.
  3. As compared to heat lost by milk to surrounding while cooling it, a very small amount of heat also flows from surrounding to milk. Hence, it is correct.
  4. By conduction, convection, and radiation, when hot milk is spread on a table, it transfers, heat to the surroundings. Hence it is correct.

 

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