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.
Calculate the temperature which has numeral value on Celsius and Fahrenheit scale.
Let $Q$ be the value of temperature having same value an Celsius and Fahrenheit scale.
Now, we can write
$$\begin{aligned} \frac{{ }^{\circ} F-32}{180} & =\frac{{ }^{\circ} \mathrm{C}}{100} \\ \Rightarrow \text{Let}\quad F & =C=Q \\ \Rightarrow \quad\frac{Q-32}{180} & =\frac{Q}{100}=Q=-40^{\circ} \mathrm{C} \text { or }-40^{\circ} \mathrm{F} \end{aligned}$$
These days people use steel utensiles with copper bottom. This is supposed to be good for uniform heating of food. Explain this effect using the fact that copper is the better conductor.
As copper is a good conductor of heat as compared to steel. The steel utensils with copper bottom absorbs heat more quickly than steel and give it to the food in utensil. As a result, of it, the food in utensil is heated uniformly and quickly.
