1.512 calories of radiation is incident on a body. It absorbs 220 calories the coefficient of emission is (MHT-CET 2001) (a) 0.43 (b) 0.57 (c) 1.7 (d) 0.34 Answer: (b) Heat energy received by the earth from the sun is due to (CPMT 1994) (a) Convection (b) Radiation (c) Reflection of light (d) Transmission of light Answer: (b) Two sphere of same material have radii r1 and r2. they are heated to same temperature and kept in same enclosure at low temperature their rates of loss of heat are in the ratio (JIPMER 98) Answer: (b) (a) (b) (c) (d) Two stars A and B radiate maximum energy at 3600A and 3600A respectively. Then the ratio of absolute temperatures of A and B is (P.M.T. MP 91) (a) 256: 81 (b) 81: 256 (c) 3: 4 (d) 4: 3 Answer: (d) Emissivity of perfectly black body is (MHT-CET 2003) (a) 1 (b) 2 (c) 5 (d) 0 Answer: (a) Absorptive power of perfectly black body is (PMT-89) (a) Zero (b) Infinity (c) One (d) Constant Answer: (c) Newton’s law of cooling is also applicable to (NCERT) (a) Convection losses (b) Natural convection losses (c) Forced convection losses (d) None of the above Answer: (c) if the surface temperature of the sun were to drop by a factor of 2. The temperature of the surrounding is T2K. the heat in the surrounding due to radiation is proportional to (NCERT 72) (a) (T14 T24) (b) (T1 T2)4 (c) (T14 T24) (d) (T13 T23) Answer: (a) If the temperature of the sun is doubled. the time taken to cool from 70C to 60C is (CPMT 92) (a) Less than 5 minute (b) 5 minute (c) More than 5 minutes (d) Less or more than 5 minutes depending upon the nature of the body Answer: (c) A sphere. the average radiant energy incident on earth per second would reduce by factor of (MHTCET 2001) (a) 2 (b) 4 (c) 8 (d) 16 A metal piece heated to T1 K.94) (a) 2 (b) 4 (c) 8 (d) 16 Answer: (d) A body in a room cools from 90C to 80C in 5 minutes. The rate of energy received on the earth will be increased by a factor of (CBSE93. a cube and a thin circular plate. which of these will cool fastest? (MNR 83) (a) Sphere (b) Cube (c) Plate (d) None Answer: (c) . all made of the same material and having the same mass are initially heated to a temperature of 3000K.Which of the following statement is wrong? (NCERT 76) (a) Rough surfaces are better radiators tan smooth surfaces (b) Highly polished mirror surfaces are very good radiators (c) Black surfaces are better absorbers then white ones (d) Black surfaces are better radiators then white ones Answer: (b) The best ideal black body is (CPMT-86) Answer: (d) (a) Lamp of charcoal heated to high temperature (b) Metal coated with a black dye (c) Glass surface coated with coalter (d) Hollow enclosure blackened inside and having a small hole The earth intercepts approximately one billionth of the power Answer: (d) radiated by the sun. it follows from the above data that its surface temperature is.The temperature of a piece of metal is raised from 27C to 51.2C.(I. coefficient of absorption and coefficient transmission are related as (MHT-CET 2005) (a) a r 1 1 (b) a r t 1 (c) a r t (d) a r 1 Answer: (a) The earth receives at its surface radiation from the sun at the rate of 1400 W/m2. (MP-PMT-95. at what temperature would it radiant energy twice as first? The temperature of surrounding may be assumed to be negligible (PMT-MP 90) (a) 200C (b) 200K (c) 800C (d) 800K Answer: (d) A body at high temperature T K radiates heat at rate proportional to(MHT CET 2001) (a) T4 (b) T4 (c) T (d) T2 Answer: (a) Co-efficient of reflection. treating sun as a black body.36 times (c) 3. the distance of the centre of the sun from the surface of the earth is 1. MHT-CET 2005) (a) Line spectrum (b) Band spectrum (c) Continuous spectrum (d) None of these Answer: (c) An object is at the temperature of 400C.5 1011 m and the radius of the sun is 7. in cooling from 60C to 50C. The ambient temperature is 30C.36 times (b) 2.T 89) (a) 5801 K (b) 106 K Answer: (a) . the rate at which metal radiates energy increases nearly (MNR 87) (a) 1. MHT-CET 2002) (a) 50 sec (b) 90 sec (c) 60 sec (d) 30 sec Answer: (a) Spectrum of a perfectly black body is (MP PMT-89. it will take.0 108 m.36 times Answer: (a) A cup of tea cools from 80C to 60C in one minute.36 times (d) 4.I. The surrounding temperature is (MHTCET-1999) (a) 30C (b) 28C (c) 36C (d) 32C Answer: (a) According to Wien’s law (PMT MP 88) (a) m T constant Answer: (a) . the maximum temperature attained must be (Wien’s constant 2.02 (b) 0.4 (d) 0.1 K (d) 2801C The process of heat transfer in which heat is transferred with actual migration of medium particles is known as (AFMC-94) (a) Conduction (b) Convection (c) Radiation (d) Reflection Answer: (b) A black body is at a temperature of 500K. then the value of t is (MHT-CET 2003) (a) 0.2 Answer: (b) A body cools from 50C to 46C in 5 minutes and to 40C in the next 10 minutes.24. r 0.86 107 K (b) 1013 K (c) 107 K (d) 107 K Answer: (d) If a 0.93 1010m.04 (c) 0.72. it emits energy at a Answer: (d) rate which is proportional to (CBSE 97) (a) 500 (b) (500)2 (c) (500)3 (d) (500)4 Which of the following is most repaid process (NCERT 80) (a) Conduction (b) Convection (c) Radiation (d) None of these Answer: (c) The wavelength of maximum energy released during an atomic explosion was 2.93 103mK) (MHT-CET 2002) (a) 5.(c) 50. it emits energy at a rate which is proportional to (AIMS-2002) (a) (300) (b) (300)2 (c) (300)4 (d) (300)3 Answer: (c) Which of the following will radiate heat to large extent? (MNR-92) (a) Rough surface (b) Polished surface (c) Black rough surface (d) Black polished surface Answer: (c) The rate of radiation of black body at 0C is E watt. AIMS 98) (a) Is a bad conductor (b) Increases the temperature of body (c) Decreases the temperature (d) All of these Answer: (a) The good absorber of heat are (PMT 89) (a) Non-emitter (b) Poor-emitter (c) Good-emitter (d) Highly polished Answer: (c) . CPMT 88. because wool (EAMCET 78. the radiant energy will become (MPPMT 92. MNR 93) Answer: (c) (a) (b) (c) (d) None of the above Woolen clothes keep the body warm. The n the rate of radiation of this black body at 273C will be (MP-PMT 89) (a) 16 E (b) 8 E (c) 4 E (d) E Answer: (a) A black body radiates energy at the rate of E watt m2 at a temperature of T Kelvin.(b) constant (c) constant (d) T m constant A black body is at 300 K. When the temperature is reduced to T/2 Kelvin. the power received by surface will becomes approximately (NSEP 89) (a) P (b) 2 P (c) 4 P (d) 16 P Answer: (c) If temperature of a black body increases from 7C to 287C.5 105 cm.T 88) (a) Greater then by the second (b) Less than that by the second (c) Equal in both cases (d) The information is incomplete to draw and conclusion Answer: (c) A surface at temperature T0 K receives power P by radiation from a small sphere at temperature T T0 and at a distance d. if both T and d are doubled. the temperature of the source (on Kelvin scale) will be n times the temperature of another source on (Kelvin scale) for which the wavelength at a maximum energy is 5.I. according to Wien’s law. CBSE 95. then (NCERT 80. MHT-CET 99) (a) t1 t2 t3 (b) t1 t2 t3 (c) t1 t2 t3 (d) t1 t2 t3 The velocity with which thermal radiation travels in vacuum is(EAMCET-82) (a) 3 106 m/s (b) 3 107 m/s (c) 3 108 m/s Answer: (c) . then the rate of energy radiation becomes (HARYANA PMT-2000) Answer: (b) (a) (b) 16 times (c) 4 times (d) 2 times A bucket full of hot water is kept in a room and it cools from Answer: (b) 75C to 70C in t1 minutes from 70C to 65C in t2 minutes and from 65C to 60C in t3 minutes.The maximum energy in the thermal radiation from a hot source occurs at a wavelength of 11 105 cm. The value of n is(CPMT 91) (a) 2 (b) 4 (c) 1/2 (d) 1 Answer: (a) Two sphere of the same material have radii 1 m and 4 m and temperature 4000 K and 2000 K respectively. the energy radiated per second by the first sphere is (I. 5C (b) 40C (c) 42. If the room temperature is 25C and assuming Newton’s law of cooling to hold good. then what is the ratio of heat radiation energy emitted per second by them? (MHT-CET 2004) (a) 1: 4 (b) 4: 1 (c) 3: 4 (d) 4: 3 Answer: (b) Solar radiation emitted by sun resembles that emitted by a black body at a temperature of 6000 K. maximum intensity is emitted at a wavelength of about 4800 A.(d) 3 1016 m/s In which process the rate of transfer of heat is maximum? (CMEE 94) (a) Conduction (b) Convection (c) Radiation (d) In all these heat is transferred with the same velocity Answer: (c) A body cools from 60C to 50C in 10 minutes. the temperature of the body at the end of the next 10 minutes will be (MP-PMT 98) (a) 38. its temperature T2. then the peak intensity would occur at a wavelength of (PMT MP 91) (a) 4800 A (b) 9600 A (c) 2400 A (d) 19200 A Answer: (b) A perfectly black body emits radiation at temperature T1 K. will be (BHU 94) (a) T2 16 T1 (b) T2 8 T1 (c) T1 4 T1 (d) T2 2 T1 Answer: (d) Unit of Stefan’s constant is given by (MHT-CET 2006) (a) W/ m K2 (b) W/ m2 K2 (c) W2/ m2 K4 (d) W/ mK Answer: (b) A black body at hot temperature at 227C radiates heat at a rate of 5 cal/cm2 s.85C (d) 45C Answer: (c) Two spheres made of same material have radii in the ratio 2:1. if it sis to radiate 16 times this power. if both the spheres are at same temperature. if the sun were cooled down from 6000 K to 3000 K. at a temperature of 727C the rate of heat radiated per unit area will be Answer: (c) . the thermal conduction is maximum? (AFMC PUNE-2002) (a) Combustion (b) Radiation (c) Convection (d) Conduction Answer: (b) .(MHT-CET 2002) (a) 50 cal/cm2 s (b) 250 cal/cm2 s (c) 80 cal/cm2 s (d) 100 cal/cm2 s According to Kirchhoff’s law of radiation which of the following Answer: (b) relation is correct (MHT CET 2006) (a) Eb Ea (b) (c) Eb E2a (d) Eb E a A person with dark skin as compared to a person with white skin will experience (CPMT-88) (a) Less heat and less cold (b) More heat and more cold (c) More heat and less cold (d) Less heat and more cold. For which of the following process.