Heat Transfer PYQs Set 7 (PDF)

Set 7 of 7 · Questions 181-209 of 209

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Mere Teacher · Previous Year Questions JEE Main Physics · Heat Transfer · Set 7 (Q181-209)
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181. $The cause of Fraunhoffer lines is$ (1) $Reflection of radiations by chromosphere$ (2) $Absorption of radiations by chromosphere$ (3) $Emission of radiations by chromosphere$ (4) $Transmission of radiations by chromosphere$ 182. $Assuming the Sun to be a black body of radius R and surface temperature T, the total radiant power incident on Earth, at a distance r from the Sun, is (earth radius = r_{0})$ (1) $\frac{R ^{2} σ T ^{4}}{r ^{2}}$ (2) $\frac{4 π r _{0}^{2} R ^{2} σ T ^{4}}{r ^{2}}$ (3) $\frac{π r _{0}^{2} R ^{2} σ T ^{4}}{r ^{2}}$ (4) $\frac{r _{0}^{2} R ^{2} σ T ^{4}}{4 π r ^{2}}$ 183. $Colour of shining bright star is an indication of its$ (1) $Distance from the earth$ (2) $Size$ (3) $Temperature$ (4) $Mass$ 184. $In which process, the rate of transfer of heat is maximum$ (1) $Conduction$ (2) $Convection$ (3) $Radiation$ (4) $In all these, heat is transferred with the same velocity$ 185. $Absorption co-efficient of an open window is...$ (1) $Zero$ (2) $0.5$ (3) $1$ (4) $0.25$ 186. $Two rectangular blocks, having indentical dimensions, can be arranged either in configuration I or in configuration II as shown in the figure, On of the blocks has thermal conductivity k and the other 2k. The temperature difference between the ends along the x-axis is the same in both the configurations. It takes 9s to transport a certain amount of heat from the hot end to the cold end in the configuration I. The time to transport the same amount of heat in the configuration II is:$ (1) $2.0 s$ (2) $3.0 s$ (3) $4.5 s$ (4) $6.0 s$ 187. $A hot and a cold body are kept in vacuum separated from each other. Which of the following cause decrease in temperature of the hot body$ (1) $Radiation$ (2) $Convection$ (3) $Conduction$ (4) $Temperature remains unchanged$ 188. $The velocity of heat radiation in vacuum is$ (1) $Equal to that of light$ (2) $Less than that of light$ (3) $Greater than that of light$ (4) $Equal to that of sound$ 189. $Two materials having coefficients of thermal conductivity \cdot 3 K^{’} and^{’} K^{’} and thickness^{’} d^{’} and ' 3 d', respectively, are joined to form a slab as shown in the figure. The temperatures of the outer surfaces are \cdot^{’} θ_{2}^{’} and^{’} θ_{1}^{’} respectively, (θ_{2} > θ_{1}) . The temperature at the interface is:$ (1) $\frac{θ _{1}}{10} + \frac{9 θ _{2}}{10}$ (2) $\frac{θ _{2} + θ _{1}}{2}$ (3) $\frac{θ _{1}}{6} + \frac{5 θ _{2}}{6}$ (4) $\frac{θ _{1}}{3} + \frac{2 θ _{2}}{3}$ 190. $Three rods of identical cross-section and lengths are made of three different materials of thermal conductivity K_{1}, K_{2} and K_{3} respectively. They are joined together at their ends to make a long rod (see figure). One end of the long rod is maintained at 100^{\circ} C and the other at 0^{\circ} C (see figure). If the joints of the rod are at 70^{\circ} C and 20^{\circ} C in steady state and there is no loss of energy from the surfac of the rod, the correct relationship between K_{1}, K_{2} and K_{3} is$ (1) $K_{1} : K_{3} = 2 : 3, K_{2} : K_{3} = 2 : 5$ (2) $K_{1} < K_{2} < K_{3}$ (3) $K_{1} : K_{2} = 5 : 2, K_{1} : K_{3} = 3 : 5$ (4) $K_{1} > K_{2} > K_{3}$ 191. $Three rods of Copper, brass and steel are welded together to form a Y-shaped structure. Area of cross-section of each rod = 4 c m^{2} . End of copper rod is maintained at 100°C where as ends of brass and steel are kept at 0°C. Lengths of the copper, brass and steel rods are 46, 13 and 12 cm respectively. The rods are thermally insulated from surroundings except at ends. Thermal conductivities of copper, brass and steel are 0.92, 0.26 and 0.12 CGS units respectively. Rate of heat flow through copper rod is:$ (1) $1.2 cal/s$ (2) $2.4 cal/s$ (3) $4.8 cal/s$ (4) $6.0 cal/s$ 192. $The heat capacities of three liquids A, B and C of same volumes are in the ratio 3 : 2 : 1. They are allowed to cool in the same surroundings and in same conditions for the same temperature difference. Which of these will cool first?$ (1) $A$ (2) $B$ (3) $C$ (4) $All will cool in same time$ 193. $There is a black spot on a body. If the body is heated and carried in dark room then it glows more. This can be explained on the basis of$ (1) $Newton’s law of cooling$ (2) $Wien’s law$ (3) $Kirchoff’s law$ (4) $Stefan’s$ 194. $If the temperature of the sun (black body) is doubled, the rate of energy received on earth will be increased by a factor of$ (1) $2$ (2) $4$ (3) $8$ (4) $16$ 195. $Relation between emissivity e and absorptive power a is (for black body)$ (1) $e = a$ (2) $e = \frac{1}{a}$ (3) $e = a^{2}$ (4) $a = e^{2}$ 196. $Heat travels through vacuum by$ (1) $Conduction$ (2) $Convection$ (3) $Radiation$ (4) $Both (a) and (b)$ 197. $Good absorbers of heat are$ (1) $Poor emitters$ (2) $Non-emitters$ (3) $Good emitters$ (4) $Highly polished$ 198. $Infrared radiation is detected by$ (1) $Spectrometer$ (2) $Pyrometer$ (3) $Nanometer$ (4) $Photometer$ 199. $A thermos flask is polished well$ (1) $To make attractive$ (2) $For shining$ (3) $To absorb all radiations from outside$ (4) $To reflect all radiations from outside$ 200. $Pick out the statement which is not true$ (1) $IR radiations are used for long distance photography$ (2) $IR radiations arise due to inner electron transitions in atoms$ (3) $IR radiations are detected by using a bolometer$ (4) $Sun is the natural source of IR radiation$ 201. $If a black body is heated at a high temperature, it seems to be$ (1) $Blue$ (2) $White$ (3) $Red$ (4) $Black$ 202. $When the body has the same temperature as that of surroundings$ (1) $It does not radiate heat$ (2) $It radiates the same quantity of heat as it absorbs$ (3) $It radiates less quantity of heat as it receives from surroundings$ (4) $It radiates more quantity of heat as it receives heat from surroundings$ 203. $How is the temperature of stars determined by$ (1) $Stefan’s law$ (2) $Wein’s displacement law$ (3) $Kirchhoff’s law$ (4) $Ohm’s law$ 204. $A body, which emits radiations of all possible wavelengths, is known as$ (1) $Good conductor$ (2) $Partial radiator$ (3) $Absorber of photons$ (4) $Perfectly black-body$ 205. $At a certain temperature for given wave length, the ratio of emissive power of a body to emissive power of black body in same circumstances is known as$ (1) $Relative emissivity$ (2) $Emissivity$ (3) $Absorption coefficient$ (4) $Coefficient of reflection$ 206. $The value of Stefan’s constant is$ (1) $5.67 \times 1 0^{- 8} W / m^{2} - K^{4}$ (2) $5.67 \times 1 0^{- 5} W / m^{2} - K^{4}$ (3) $5.67 \times 1 0^{- 11} W / m^{2} K^{4}$ (4) $None of these$ 207. $A metal is heated in a furnace where a sensor is kept above the metal surface to read the power radiated (P) by the metal. the sensor has scale that displays l o g_{2} (P / P_{0}), where P_{0} is a constant. When the metal surface is at a temperature of 487 ºC, the sensor shows a value 1. Assume that the emissivity of the metallic surface remains constant. What is the value displayed by the sensor when the temperature of the metal surface is raised to 2767 ºC.$ 208. $A metal rod AB of length 10 x has its one end A in ice at 0ºC and the other end B in water at 100ºC. If a point P on the rod is maintained at 400ºC, then it is found that equal amounts of water and ice evaporate and melt per unit time. The latent heat of evaporation of water is 540 cal/g and latent heat of melting of ice is 80 cal/g. If the point P is at a distance of λx from the ice end A, find the value of λ. [Neglect any heat loss to the surrounding]$ 209. $Two conducting cylinders of equal length but different radii are connected in series between two heat baths kept at temperatures T_{1} = 300K and T_{2} = 100K, as shown in the figure. The radius of the bigger cylinder is twice that of the smaller one and the thermal conductivities of the materials of the smaller and the larger cylinders are K_{1} and K_{2} respectively. If the temperature at the junction of the two cylinders in the steady state is 200K, then K_{1} / K_{2} =__________.$