Heat Transfer PYQs Set 3 (PDF)

Set 3 of 7 · Questions 61-90 of 200

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Mere Teacher · Previous Year Questions NEET Physics · Heat Transfer · Set 3 (Q61-90)
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61. $A piece of iron is heated in a flame. It first becomes dull red then becomes reddish yellow and finally turns to white hot. The correct explanation for the above observation is possible by using:$ (1) $Wien’s displacement Law$ (2) $Kirchoff’s Law$ (3) $Newton’s Law of cooling$ (4) $Stefan’s Law$ 62. $If the temperature of a black body increases from 7^{\circ} C to 287^{\circ} C, then rate of energy radiation is$ (1) $16 times$ (2) $8 times$ (3) $4 times$ (4) $2 times$ 63. $The temperature of a room is 30°C. A body kept in it, takes 4 minutes in cooling from 61°C to 59°C. The time taken by the body in cooling from 51°C to 49°C will be-$ (1) $4 min.$ (2) $5 min.$ (3) $6 min.$ (4) $8 min.$ 64. $A black body at a temperature of 1640 K has the wavelength corresponding to maximum emission equal to 1.75 μ . Assuming the moon to be a perfectly black body, the temperature of the moon, if the wavelength corresponding to maximum emission is 14.35 μ is$ (1) $100 K$ (2) $150 K$ (3) $200 K$ (4) $250 K$ 65. $A body cools from 60°C to 50°C in 10 minutes. If the room temperature is 25°C and assuming newton's law of cooling to hold good, the temperature of the body at the end of the next 10 minutes will be:$ (1) $45°C$ (2) $42.8°C$ (3) $40°C$ (4) $38.5°C$ 66. $A cup of tea cools from 80 ℃ to 60 ℃ in one minute. The ambient temperature is 30 ℃ . In cooling from 60 ℃ to 50 ℃ it will take$ (1) $30 Seconds$ (2) $60 Seconds$ (3) $90 Seconds$ (4) $50 Seconds$ 67. $A body cools from 50. 0^{o} C to 49. 9^{o} C in 5 s . How long will it take to cool from 40. 0^{o} C to 39. 9^{o} C ? Assume the temperature of surroundings to be 30. 0^{o} C and Newton's law of cooling to be valid$ (1) $2.5 s$ (2) $10 s$ (3) $20 s$ (4) $5 s$ 68. $A spherical black body with a radius of 12 cm radiates 450 watt power at 500 K. If the radius were halved and the temperature doubled, the power radiated in watt would be$ (1) $225$ (2) $450$ (3) $1000$ (4) $1800$ 69. $A black body at 200 K is found to exit maximum energy at a wavelength of 14μm. When its temperature is raised to 1000K, the wavelength at which maximum energy is emitted is$ (1) $14μm$ (2) $70μF$ (3) $2.8μm$ (4) $2.8mm$ 70. $An object is at a temperature of 400 ℃ . At what temperature would it radiate energy twice as fast? The temperature of the surroundings may be assumed to be negligible$ (1) $200 ℃$ (2) $200 K$ (3) $800 ℃$ (4) $800 K$ 71. $Two identical objects A and B are at temperatures T_{A} and T_{B} respectively. Both objects are placed in a room with perfectly absorbing walls maintained at temperatures T (T_{A} > T > T_{B}) . The objects A and B attain temperature T eventually which one of the following is correct statement$ (1) $‘A’ only emits radiations while B only absorbs them until both attain temperature$ (2) $A loses more radiations than it absorbs while B absorbs more radiations that it emits until temperature T is attained$ (3) $Both A and B only absorb radiations until they attain temperature T$ (4) $Both A and B only emit radiations until they attain temperature T$ 72. $What will be the ratio of temperatures of sun and moon if the wavelengths of their maximum emission radiations rates are 140 Å and 4200 Å respectively$ (1) $1 : 30$ (2) $30 : 1$ (3) $42 : 14$ (4) $14 : 42$ 73. $Two thermometers A and B are exposed in sun light. The valve of A is painted black, But that of B is not painted. The correct statement regarding this case is$ (1) $Temperature of A will rise faster than B but the final temperature will be the same in both$ (2) $Both A and B show equal rise in beginning$ (3) $Temperature of A will remain more than B$ (4) $Temperature of B will rise faster$ 74. $A metal ball of surface area 200 c m^{2} and temperature 52 7^{o} C is surrounded by a vessel at 2 7^{o} C . If the emissivity of the metal is 0.4, then the rate of loss of heat from the ball is (σ = 5.67 \times 1 0^{- 8} J / m^{2} - s - k^{4})$ (1) $108 joules approx.$ (2) $168 joules approx.$ (3) $182 joules approx.$ (4) $192 joules approx.$ 75. $Which of the following law states that “good absorbers of heat are good emitters”$ (1) $Stefan’s law$ (2) $Kirchoff’s law$ (3) $Planck’s law$ (4) $Wein’s law$ 76. $The maximum energy in thermal radiation from a source occurs at the wavelength 4000Å. The effective temperature of the source is (Wien's constant, b = 2.93 \times 10^{- 3} m K)$ (1) $7325 K$ (2) $80000 K$ (3) $1 0^{4} K$ (4) $1 0^{6} K$ 77. $For a perfectly black body, its absorptive power is$ (1) $1$ (2) $0.5$ (3) $0$ (4) $Infinity$ 78. $A bucket full of hot water cools from 7 5^{o} C to 7 0^{0} C in time T_{1}, from 7 0^{o} C to 6 5^{o} C in time T_{2} and from 6 5^{o} C to 6 0^{o} C in time T_{3}, then$ (1) $T_{1} = T_{2} = T_{3}$ (2) $T_{1} > T_{2} > T_{3}$ (3) $T_{1} < T_{2} < T_{3}$ (4) $T_{1} > T_{2} < T_{3}$ 79. $If the sun’s surface radiates heat at 6.3 \times 1 0^{7} W m^{- 2} . Calculate the temperature of the sun assuming it to be a black body (σ = 5.7 \times 1 0^{- 8} W m^{- 2} K^{- 4})$ (1) $5.8 \times 1 0^{3} K$ (2) $8.5 \times 1 0^{3} K$ (3) $3.5 \times 1 0^{8} K$ (4) $5.3 \times 1 0^{8} K$ 80. $A container contains hot water at 10 0^{o} C . If in time T_{1} temperature falls to 8 0^{o} C and in time T_{2} temperature falls to 6 0^{o} C from 8 0^{o} C, then$ (1) $T_{1} = T_{2}$ (2) $T_{1} > T_{2}$ (3) $T_{1} < T_{2}$ (4) $None$ 81. $a piece of ice falls from a height h so that it melts completely. Only one-quarter of the heat produced is absorbed by the ice and all energy of ice gets converted in to heat during its fall. The value of h is: [Latent heat of ice is 3.4 \times 1 0^{5} J/Kg and g = 10 N/kg]$ (1) $68 km$ (2) $34 km$ (3) $544 km$ (4) $136 km$ 82. $A body takes 10 minutes to cool down from 62°C to 50°C. If the temperature of surrounding is 26°C then in the next 10 minutes temperature of the body will be:$ (1) $38°C$ (2) $40°C$ (3) $42°C$ (4) $44°C$ 83. $A human body has surface area of approximately 1 m^{2} . The normal body temperature is 10 K above the surrounding room temperature T0. Take the room temperature to be T0 = 300 K. For T0 = 300K, the value of σ T_{0}^{4} = 460 W m^{-2} (where σ is the Stefan-Boltzmann constant). Which of the following options is/are correct?$ (1) $If the surrounding temperature reduces by a small amount ∆ T_{0} << T_{0}, then to maintain the same body temperature the same (living) human being needs to radiate ∆W = 4 σ T_{0}^{3} Δ T_{0} more energy per unit time.$ (2) $Reducing the exposed surface area of the body (e.g. by curling up) allows humans to maintain the same body temperature while reducing the energy lost by radiation$ (3) $If the body temperature rises significantly then the peak in the spectrum of electromagnetic radiation emitted by the body would shift to longer wavelengths$ (4) $The amount of energy radiated by the body in 1 second is close to 60 joules$ 84. $Certain quantity of water cools from 70 ℃ to 60 ℃ in the first 5 minutes and to 54 ℃ in the next 5 minutes. The temperature of the surroundings is;$ (1) $45 ℃$ (2) $20 ℃$ (3) $42 ℃$ (4) $10 ℃$ 85. $The temperature of a liquid drops from 365 K to 361 K in 2 minutes. Find the time during which temperature of the liquid drops from 344 K to 342 K . Temperature of room is 293 K$ (1) $84 sec$ (2) $72 sec$ (3) $66 sec$ (4) $60 sec$ 86. $A heat source at T = 1 0^{3} K is connected to another heat reservoir at T = 1 0^{2} K by a copper slab which is 1m thick. Given that the thermal conductivity of copper is 0.1 W K^{-1} m^{-1}, the energy flux through it in the steady state is:$ (1) $200 W m^{-2}$ (2) $90 W m^{-2}$ (3) $65 W m^{-2}$ (4) $120 W m^{-2}$ 87. $The energy spectrum of a black body exhibits a maximum around a wavelength λ_{o} . The temperature of the black body is now changed such that the energy is maximum around a wavelength \frac{3 λ _{o}}{4} .The power radiated by the black body will now increase by a factor of$ (1) $256/81$ (2) $64/27$ (3) $16/9$ (4) $4/3$ 88. $Two spherical black bodies of radii r_{1} and r_{2} and with surface temperature T_{1} and T_{2} respectively radiate the same power. Then the ratio of r_{1} and r_{2} will be$ (1) $(\frac{T _{2}}{T _{1}})^{2}$ (2) $(\frac{T _{2}}{T _{1}})^{4}$ (3) $(\frac{T _{1}}{T _{2}})^{2}$ (4) $(\frac{T _{1}}{T _{2}})^{4}$ 89. $At 12 7^{o} C radiates energy is 2.7 \times 1 0^{- 3} J/s. At what temperature radiated energy is 4.32 \times 10^{6} J/s$ (1) $400 K$ (2) $4000 K$ (3) $80000 K$ (4) $40000 K$ 90. $Distribution of energy in the spectrum of a black body can be correctly represented by$ (1) $Wien's law$ (2) $Stefan's law$ (3) $Planck's law$ (4) $Kirchhoff's law$