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?
A glass full of hot milk is poured on the table. It begins to cool gradually. Which of the following is correct?
Is the bulb of a thermometer made of diathermic or adiabatic wall?
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.
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.
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 $$\begin{aligned} \Delta l & =\alpha l \Delta T \\ & =2 \times 10^{-5} \times 3 \times 10=6 \times 10^{-4} \mathrm{~m}=6 \times 10^{-4} \mathrm{~m} \quad \text{[i.e., observed value (Correct)]} \end{aligned}$$
Why does a metal bar appear hotter than a wooden bar at the same temperature? Equivalently it also appears cooler than wooden bar if they are both colder than room temperature.
Due to difference in conductivity, metals having high conductivity compared to wood. On touch with a finger, heat from the surrounding flows faster to the finger from metals and so one feels the heat. Similarly, when one touches a cold metal the heat from the finger flows away to the surroundings faster.