Thermal Properties of Matter's Question 10 | Physics XI - Part 2 | NCERT Exemplar Questions

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MCQ (Multiple Correct Answer)

Gulab namuns (assumed to be spherical) are to be heated in an oven. They are available in two sizes, one twice bigger (in radius) than the other. Pizzas (assumed to be discs) are also to be heated in oven. They are also in two sizes, one twice bigger (in radius) than the other. All four are put together to be heated to oven temperature. Choose the correct option from the following.

Both size gulab jamuns will get heated in the same time

Smaller gulab jamuns are heated before bigger ones

Smaller pizzas are heated before bigger ones

Bigger pizzas are heated before smaller

MCQ (Multiple Correct Answer)

Refer to the plot of temperature versus time (figure) showing the changes in the state if ice on heating (not to scale). Which of the following is correct?

The region $A B$ represent ice and water in thermal equilibrium

At $B$ water starts boiling

At $C$ all the water gets converted into steam

$C$ to $D$ represents water and steam in equilibrium at boiling point

MCQ (Multiple Correct Answer)

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

The rate of cooling is constant till milk attains the temperature of the surrounding

The temperature of milk falls off exponentially with time

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

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

Subjective

Is the bulb of a thermometer made of diathermic or adiabatic wall?

Explanation

As diathermic walls allow exchange of heat energy between two systems and adiabatic walls do not, hence, diathermic walls are used to make the bulb of a thermometer.

Subjective

A student records the initial length $l$, change in temperature $\Delta T$ and change in length $\Delta l$ of a rod as follows

S. No.	$$l$$ (m)	$$\Delta T$$ ($$^\circ$$C)	$$\Delta l$$(m)
1.	2	10	$$4\times10^{-4}$$
2.	1	10	$$4\times10^{-4}$$
3.	2	20	$$2\times10^{-4}$$
4.	3	10	$$6\times10^{-4}$$

If the first observation is correct, what can you say about observation 2, 3 and 4.

Explanation

From the 1st observation $\alpha=\frac{\Delta l}{l \Delta T} \Rightarrow \alpha=\frac{4 \times 10^{-4}}{2 \times 10}=2 \times 10^{-5}{ }^{\circ} \mathrm{C}^{-1}$

For 2nd observation $$ \begin{aligned} \Delta l & =\alpha l \Delta T \\ & =2 \times 10^{-5} \times 1 \times 10=2 \times 10^{-4} \mathrm{~m} \neq 4 \times 10^{-4} \mathrm{~m} \text { (Wrong) } \end{aligned}$$

For 3rd observation $$\begin{aligned} \Delta l & =\alpha l \Delta T \\ & =2 \times 10^{-5} \times 2 \times 20=8 \times 10^{-4} \mathrm{~m} \neq 2 \times 10^{-4} \mathrm{~m}(\text { Wrong }) \end{aligned}$$

For 4th observation $$\begin{aligned} \Delta l & =\alpha l \Delta T \\ & =2 \times 10^{-5} \times 3 \times 10=6 \times 10^{-4} \mathrm{~m}=6 \times 10^{-4} \mathrm{~m} \quad \text{[i.e., observed value (Correct)]} \end{aligned}$$

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