Ncert Exercises & Solutions

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

(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 surroundings as well as from the surroundings to the milk but the net flow of heat is from milk to the surroundings and that is why it cools.

(d) All three phenomena, conduction, convection and radiation are responsible for the loss of heat from milk to the surroundings.

Choose the correct alterative/s:

1. (a, b, c)

2. (a, d)

3. (b, c, d)

4. (a, c, d)

(3) Hint: Apply the concept of Newton's law of cooling.

Step 1: Find the condition of cooling.

When hot milk spreads on the table heat is transferred to the surroundings by conduction, convection and radiation. According to Newton's law of cooling, the temperature of the milk falls off exponentially. Heat also will be transferred from surroundings to the milk but it will be lesser than what is transferred from milk to the surroundings.

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