Convection

Q. A body cools from ${60}^{\circ }\text{C}$ to ${50}^{\circ }\text{C}$ in $10$ minutes. If the room temperature is ${25}^{\circ }\text{C}$ then the temperature of the body at the end of the next $10$ minutes will be

1. ${38.50}^{\circ }\text{C}$
2. ${40.25}^{\circ }\text{C}$
3. ${42.86}^{\circ }\text{C}$
4. ${45.75}^{\circ }\text{C}$

Q. A body takes $4\text{minute}$ to cool from ${61}^{\circ }\text{C}$ to ${59}^{\circ }\text{C}$. If the temperature of the surroundings is ${30}^{\circ }\text{C}$, the time taken by the body to cool from ${51}^{\circ }\text{C}$ to ${49}^{\circ }\text{C}$ is :

1. $8\text{min.}$
2. $3\text{min.}$
3. $4\text{min.}$
4. $6\text{min.}$

Q. In which of the following processes, convection does not take place primarily?

1. Sea and Land breeze
2. Boiling of water
3. Heating air around a furnace
4. Warming of glass of bulb due to filament

Q. In $5\text{minutes}$, a body cools from $75{}^{\circ }\text{C}$ to $65{}^{\circ }\text{C}$ at room temperature of $25{}^{\circ }\text{C}$. The temperature of the body at the end of next $5\text{minutes}$ is ${}^{\circ }\text{C}$

Q. A temperature difference of $120{}^{\circ }\text{C}$ is maintained between two ends of a uniform rod $AB$ of length $2L$. Another bent rod $PQ$, of same cross-section as $AB$ and length $\frac{3L}{2}$, is connected across $AB$ (see figure). In steady state, temperature difference between $P$ and $Q$ will be close to:

1. $60{}^{\circ }\text{C}$
2. $35{}^{\circ }\text{C}$
3. $45{}^{\circ }\text{C}$
4. $75{}^{\circ }\text{C}$

Q. It is hotter for the same distance over the top of the fire than it is by the side is mainly because

1. air conducts heat upward.
2. heat is radiated upward.
3. convection takes more heat upwards.
4. convection, conduction and radiation all, contribute significantly in transferring heat

Q. $V-I$ graph of a conductor of temperature $T_1$ and $T_2$ are shown in the figure. Then, $(T_2-T_1)$ is proportional to

1. $\cot 2\theta$
2. $\tan 2\theta$
3. $\sin 2\theta$
4. $\cos 2\theta$

Q.

Transfer of heat in liquids and gases takes place by_______________

Q. 74.What is the formula of Self energy of:- (1) Solid Sphere (2) Hollow Sphere

Q. In natural convection, a heated portion of a liquid moves because:

1. Its molecular motion becomes aligned
2. Of molecular collisions within it
3. Its density is less than that of the surrounding fluid
4. Of currents of the surrounding fluid

Q. Why does a flame always point upwards?

Q.

What is the only method of heat transfer that does not require matter?

Q. A continuous flow water heater (geyser) has an electrical power rating $=2\text{kW}$ and efficiency of conversion of electrical power into heat $=80\%$. If water is flowing through the device at the rate of $100\text{cc/sec}$, and the inlet temperature is ${10}^{\circ }C$, the outlet temperature will be

1. ${12.2}^{\circ }C$
2. ${13.8}^{\circ }C$
3. ${20}^{\circ }C$
4. ${16.5}^{\circ }C$

Q. In heat transfer, which method is based on gravitation?

1. Natural convection
2. Radiation
3. All of the above
4. Conduction

Q.

Tea or milk cools faster when sugar is added to it. Why?

Q. A hollow sphere 10 cm ID and 30 cm OD of a material having thermal conductivity 50 W/mK is used as a container for a liquid chemical mixture. Its inner and outer surface temperature are ${300}^{\circ }C$ and ${100}^{\circ }C$ respectively. The heat flow rate through the sphere is _____kW.

1. 9.42

Q. When placed in air at ${30}^{\circ }C,$ the temperature of a body decreases from ${60}^{\circ }C\text{to}{50}^{\circ }C$ in ten minutes. After next ten minutes, its temperature will be-

1. Less than ${40}^{\circ }C$
2. ${40}^{\circ }C$
3. More than ${40}^{\circ }C$
4. Not definite

Q. If the temperature of the surrounding is increased keeping all other parameters constant, then the rate of heat transfer from a comparatively hotter body due to convection will

1. not change
2. increase
3. decrease
4. first increases then decreases

Q. Two parallel plates $A$ and $B$ are joined together to form a compound plate. The thickness of the plates are $4.0\text{cm}$ and $2.5\text{cm}$ respectively and the area of cross-section is $100{\text{cm}}^2$ for each plate.
The thermal conductivities are $K_A=200\text{W}{/}^{\circ }\text{C m}$ for the plate $A$ and $K_B=400\text{W}{/}^{\circ }\text{C m}$ for the plate $B$. The outer surface of the plate $A$ is maintained at ${100}^{\circ }C$ and the outer surface of the plate B is maintained at $0^{\circ }C$. Find the rate of heat flow through any cross-section.

1. $2810\text{W}$
2. $3540\text{W}$
3. $3810\text{W}$
4. $4240\text{W}$

Q.

What do you understand by the term ventilation?

Q. A body cools down from ${40}^{\circ }\text{C}$ to ${35}^{\circ }\text{C}$ in $10$ minutes and to ${30}^{\circ }\text{C}$ in another $15$ minutes. Find the temperature of the surroundings.

1. ${20}^{\circ }\text{C}$
2. ${22.5}^{\circ }\text{C}$
3. ${25}^{\circ }\text{C}$
4. ${27}^{\circ }\text{C}$

Q. A solid wood door has dimensions; height = 2m, width = 0.8m, thickness = 0.04m. Its thermal conductivity is $K=0.04J{s}^{-1}{m}^1{(}^{\circ }C{)}^{-1}$. The inside air temperature is ${20}^{\circ }C$ and outside air temperature is $-{10}^{\circ }C$. What is the rate of heat flow through the door, assuming the surface temperatures are those of the surrounding air.

1. 148 watts
2. 48 watts
3. 408 watts
4. 11. 48 watts

Q. Can we boil water inside the earth satellite by convection

1. Yes
2. No
3. Nothing can be said
4. In complete information is given

Q. Out of convection , conduction and radiation in which of the case the rate of heat transfer is the fastest and why ??

Q. Statement 1: Heat transfer by conduction and convection requires material medium.
Statement 2: Heat transfer by radiation doesn't require a material medium

1. $\text{Both statements are correct}$
2. $\text{Both statements are incorrect}$
3. $\text{Statement-1 is correct and Statement -2 is incorrect}$
4. $\text{Statement-2 is correct and Statement -1 is incorrect}$

Q. It is hotter for the same distance over the top of the fire than it is by the side is mainly because

1. air conducts heat upward.
2. heat is radiated upward.
3. convection takes more heat upwards.
4. convection, conduction and radiation all, contribute significantly in transferring heat

Q. Convection coefficient depends upon

1. $\text{Density}$
2. $\text{Specific heat}$
3. $\text{Thermal conductivity}$
4. $\text{All the above}$

Q. A spherical body having radius $0.5\text{m}$ is transfering heat to its surrounding through convection. Find the temperature difference between surrounding and the body surface, if the rate of heat transfer is $1.57\times {10}^5\text{W}$.
(Convection coefficient, $h_c=1570{\text{W/m}}^2\text{-K}$)

1. $35.8\text{K}$
2. $31.8\text{K}$
3. $38.8\text{K}$
4. $40.8\text{K}$

Q. A solid sphere of radius $4\text{cm}$ and a hollow sphere of same material and having outer radius $4\text{cm}$ and inner radius $2\text{cm}$ are heated to the same temperature and allowed to cool in the same room. The ratio of the rate of fall of temperatures is

1. $7:8$
2. $8:5$
3. $2:1$
4. $4:1$

Q.

The phenomenon due to which particles of a medium actually move to the source of heat is called:

1. radiation

2. conduction

3. convection

4. none of the above