Q. No.A pan filled with hot food cools in 2 minutes from \(94^{\circ}\mathrm{C}\) to \(86^{\circ}\mathrm{C}\) when the room temperature is \(20^{\circ}\mathrm{C}\). How long will it take to cool from \(71^{\circ}\mathrm{C}\) to \(69^{\circ}\mathrm{C}\)?
1.
50 sec
2.
52 sec
3.
42 sec
4.
48 sec
Consider a compound slab consisting of two different materials having equal thicknesses and thermal conductivities K and 2K, respectively. The equivalent thermal conductivity of the slab will be:
1.
2.
3.
4.
A cylindrical rod has temperatures at its ends. The rate of flow of heat is cal/sec. If all the linear dimensions are doubled while keeping the temperature constant, then the rate of flow of heat will be:
1.
2.
3.
4.
Which of the following is closest to an ideal black body?
| 1. | Black lamp |
| 2. | Cavity maintained at a constant temperature |
| 3. | Platinum black |
| 4. | A lump of charcoal heated to high temperature |
Two conducting slabs of heat conductivity \(K_{1} ~ \text{and} ~K_{2}\) are joined as shown in fig. If the temperature at the ends of the slabs are \(\theta_{1}\ and\ \theta_{2} \ (\theta_{1} > \theta_{2} ), \)then the final temperature \( \left(\theta\right)_{m} \)of the junction will be:
| 1. | \(\frac{K_{1} \theta_{1} + K_{2} \theta_{2}}{K_{1} + K_{2}}\) | 2. | \(\frac{K_{1} \theta_{2} + K_{2} \theta_{1}}{K_{1} + K_{2}}\) |
| 3. | \(\frac{K_{1} \theta_{2} + K_{2} \theta_{1}}{K_{1} - K_{2}}\) | 4. | None |
A slab of stone with an area \(0.36~\text{m}^{2}\) and thickness of \(0.1~\text{m}\) is exposed on the lower surface to steam at \(100^{\circ}\mathrm{C}\). A block of ice at \(0^{\circ}\mathrm{C}\) rests on the upper surface of the slab. In one hour \(4.8~\text{kg}\) of ice is melted. The thermal conductivity of the slab will be: (Given latent heat of fusion of ice \(= 3.36\times10^{5}~\text{JKg}^{-1}\))
1. \(1.29~\text{J/m/s/}^{\circ}\text{C}\)
2. \(2.05~\text{J/m/s/}^{\circ}\text{C}\)
3. \(1.02~\text{J/m/s/}^{\circ}\text{C}\)
4. \(1.24~\text{J/m/s/}^{\circ}\text{C}\)
Three stars \(A,\) \(B,\) and \(C\) have surface temperatures \(T_A,~T_B\)and \(T_C\) respectively. Star \(A\) appears bluish, star \(B\) appears reddish and star \(C\) yellowish. Hence,
1. \(T_A>T_B>T_C\)
2. \(T_B>T_C>T_A\)
3. \(T_C>T_B>T_A\)
4. \(T_A>T_C>T_B\)
A piece of iron is heated in a flame. If it becomes dull red first, then becomes reddish yellow, and finally turns to white hot, the correct explanation for the above observation is possible by using:
| 1. | Stefan's law | 2. | Wien's displacement law |
| 3. | Kirchhoff's law | 4. | Newton's law of cooling |
The diagram shows a bimetallic strip used as a thermostat in a circuit. Copper expands more than Invar for the same temperature rise.
What will be switched on when the bimetallic strip becomes hot?
| 1. | bell only | 2. | lamp and bell only |
| 3. | motor and bell only | 4. | lamp, bell, and motor |
| Assertion (A): | While measuring the thermal conductivity of liquid experimentally, the upper layer is kept hot and the lower layer is kept cold. |
| Reason (R): | This avoids the heating of liquid by convection. |
| 1. | Both (A) and (R) are true and (R) is the correct explanation of (A). |
| 2. | Both (A) and (R) are true but (R) is not the correct explanation of (A). |
| 3. | (A) is true but (R) is false. |
| 4. | Both (A) and (R) are false. |
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