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6. HEAT RADIATION

1) The tungsten filament of an electric lamp has a surface area A and a power rating

P. If the emissivity of the filament is and is Stefans constant, the steady temperature of the filament will be

A) T = 2

AP

B) T = AP

C) T = 2/1

AP

D) T = 4/1

AP

2) Which one of the following statements is not true about thermal radiations?

A) All bodies emit thermal radiations at all temperatures B) Thermal radiations are electromagnetic waves C) Thermal radiations are not reflected from a mirror D) Thermal radiations travel in free space with a velocity of 3 x 10 8 ms 1

3) The amount of energy radiated by a body depends upon

A) The nature of its surface B) The area of its surface C) The temperature of its surface D) All the above factors

4) The wavelength of the radiation emitted by a body depends upon

A) The nature of its surface B) The area of its surface C) The temperature of its surface D) All the above factors

5) A solid sphere and a hollow sphere of the same material and size are heated to the

same temperature and allowed to cool in the same surroundings. If the temperature an each sphere is T, then A) The hollow sphere will cool at a faster rate for all values of T B) The solid sphere will cool at a faster rate for all values of T C) Both spheres will cool at the same rate for all values of T D) Both spheres will cool at the same rate only for small values of T.

6) If the temperature of a black body increases from 7 0 C to 287 0 C, then the rate of energy radiation increases by

A) 4

7287

B) 16 C) 4 D) 2

7) A body cools from 60 0 C to 50 0 C in 10 minutes. If the room temperature is 25 0 C

and assuming Newtons law of cooling to hold good, the temperature of the body at the end of next 10 minutes will be A) 38.5 0 C B) 40 0 C C) 42.85 0 C D) 45 0 C

8) Two rods of the same length and material transfer a given amount of heat in 12

seconds when they are joined end to end. But when they are joined lengthwise, they will transfer the same amount of heat, in the same conditions, in A) 24 s B) 3 s C) 1.5 s D) 48 s

9) A body cools from 50.0 0 C to 49.9 0 C in 5s. How long will it take to cool from

40.0 0 C to 39.9 0 C ? Assume the temperature of the surroundings to be 30.0 0 C and Newtons law of cooling to be valid. A) 2.5 s B) 10 s C) 20 s D) 5 s

10) Two metallic spheres S 1 and S 2 are made of the same material and have identical

surface finish. The mass of S 1 is three times that of S 2 . Both the spheres are heated to the same high temperature and placed in the same room having lower temperature but are thermally insulated from each other. The ratio of the initial rate of cooling of S 1 to that of S 2 is

A) 31

B) 3

1

C) 13

D) 3/1

31

11) A spherical black body of radius 12 cm radiates 450 W power at 500 K. If the

radius were halved and the temperature doubled, the power radiated in watt would be A) 225 B) 450 C) 900 D) 1800

12) A black body is at a temperature of 2880 K. The energy of radiation emitted by

this body between wavelengths 499 nm and 500 nm is U 1 , between 999 nm and 1000 nm is U 2 and between 1499 nm and 1500 nm is U 3 . The Wiens constant b=2.88 x 10 6 nm K. Then A) U 1 = 0 B) U 3 = 0 C) U 1 > U 2 D) U 2 > U1

13) When the temperature of a black body increases, it is observed that the

wavelength corresponding to maximum energy changes from 0.26 m to 0.13 m. The ratio of the emissive powers of the body at the respective temperatures is

A) 1

16

B) 14

C) 41

D) 161

14) If the temperature of the sun were to increase from T to 2T its radius from R to

2R, then the ratio of the radiant energy received on earth to what it received previously will be A) 4 B) 16 C) 32 D) 64

15) A black body emits radiations of maximum intensity at a wavelength of 50000A ,

when the temperature of the body is 1227 0 C. If the temperature of the body is increased by 1000 0 C, the maximum intensity would be observed at

A) 2754.8 0A

B) 3000 0A

C) 3500 0A

D) 4000 0A

16) Energy is being emitted from the surface of a black body at 127 0 C temperature at

the rate of 1.0 10 4 2sec

xmetrejoul . Temperature of the black body at which the

rate of energy emission is 16.00 10 4 J/s metre 2 will be A) 254 0 C B) 508 0 C C) 527 0 C D) 727 0 C

17) An electric heater, assumed to be a black body, has a temperature of 727 0 C. If its temperature is raised to 1727 0 C, the amount of energy radiated per unit time now as compared with that in the first case will be: A) twice B) four times C) 16 times D) 100 times

18) A monochromatic source of light is radiating 3.3 watt power with wavelength

6.0 10 7 metre. Then the frequency of radiation is A) 5.0. x 10 16 Hz B) 3.3 x 10 19 Hz C) 10 16 Hz D) 10 19 Hz

19) A monochromatic source of light is radiating 3.3 watt power with wavelength

6.0 10 7 metre. Then the number of photons emitted per second is A) 5 1016 B) 3.3 10 19 C) 10 19 D) 10 16

20) With cold wind keeping the surface of ice at -20 0 C, a layer of ice on a pond

grows in thickness from 20 mm to 21 mm in ten minutes. Later with the surface at the same temperature it will grow from 40 mm to 42 mm in approximately A) 10 minutes B) 10.2 minutes C) 20 minutes D) 40 minutes

21) The velocity of heat radiation in vacuum is

A) equal to that of light B) less than that of light C) greater than that of light D) equal to that of sound

22) Green houses are provided with glass doors and glass roofs because glass is

opaque to A) conduction B) convection C) radiation D) none of the above

23) Thermos flask prevents heat loss by A) conduction B) convection C) radiation D) all of the above

24) The temperature difference between a block of copper and its surrounding decays

exponentially with time. Its initial temperature at start is 40 0 C above the surroundings. Two minutes later it is 20 0 C above the surroundings. The time in minutes from the start when the temperature falls to 2.5 0 C above the surroundings is A) 4 B) 8 C) 16 D) 32

25) When the temperature of a body is increased, then the frequency corresponding to

maximum energy of radiation A) increases B) decreases C) remains unchanged D) first increases and then decreases

26) Which of the following statements is wrong

A) Stefans constant is the same for all black bodies B) Thermal radiations travel with a speed of 3 10 8 ms 4 C) Thermal radiations exhibit diffraction D) Stefans law is applicable for all heated bodies

27) An electric heater, kept in vacuum is heated continuously by passing electric

current. Its temperature will A) go on rising with time B) stop rising after some time as it will lose heat to the surroundings by

conduction C) will rise for some time and there after will start falling D) will become constant after some time due to loss of heat by radiation.

28) The spectrum from a black body radiation is a

A) line spectrum B) band spectrum C) continuous spectrum D) line and band spectrum

29) At a specific temperature, the energy density for a black body for three different wavelengths are 10, 19 and 7 units. For these wavelengths if the absorption coefficient of a body is respectively 0.8, 0.3 and 0.9, then the emissive powers of this body for these wavelengths are in the ratio A) 10: 19: 7 B) 8: 3: 9 C) 8 : 5.7 : 6.3 D) 8 : 95 : 4.3

30) A body in a room cools from 85 0 C to 80 0 C in 9 minutes. The time taken to cool

from 80 0 C to 75 0 C is A) 9 minutes B) less than 9 minutes C) more than 9 minutes D) either less or more than five minutes.

31) The bulbs of two identical thermometers are coated, one with lamp black and the

other with silver. Both are exposed to the sun and their readings are recorded A) both indicate same temperature B) The lamp-black coated thermometer gives higher reading initially but final

temperature is same C) The silver coated gives higher reading initially but final temperature is

same D) Both will show different but lower temperatures than the actual.

32) It is hotter over the top of the fire than at the same distance on the sides because

of A) conduction of heat B) convection of heat C) radiation of heat D) all of the above

33) The temperature of filament of a bulb is 500 K and amount of heat radiated is E.

If the filament is supposed to be a black body, the amount of heat radiated when the temperature of filament is 1000 K is A) 16 E B) 2 E C) 4 E D) 8 E

34) A metal piece is heated up to T K and the temperature of the surroundings in T 0 K. The radiation loss is proportional to A) (T T 0 )

4 B) ( )40

4 TT C) (T T 0 )

2 D) (T3 - T )30

35) The absorption coefficient of a perfectly black body is

A) zero B) less than 1 C) unity D) more than 1

36) Corresponding to a given temperature, there is a wavelength max for which the

intensity of heat radiation is A) maximum B) minimum C) zero D) constant for all temperatures

37) A black body at high temperature emits radiations of

A) large wavelength B) small wavelength C) one fixed wavelength D) all wavelengths

38) There are two spherical balls A and B of the same material with same surface

finish but the radius of A is half than that of B. If A and B are heated to the same temperature and allowed to cool then A) rate of cooling of both is same B) rate of cooling of A is four times that of B C) rate of cooling of A is twice that of B. D) rate of cooling of A is times that of B.

39) Wiens constant is 2.892 10-3 m-K and value of max from moon is 14.46

micron. What is the surface temperature? A) 100 K B) 200 K C) 400 K D) 600 K.

40) Luminosity of sun is 3.9 10 26 watts. Mean distance of earth from the sun is 1.496 1011 m. What is the value of solar constant? A) 1.388 10 5 W B) 1.388 10 3 W m 2 C) 2.776 10 6 W m 2 D) 2.776 10 4 W

41) A perfectly black body is one whose emissive power is A) zero B) unity C) maximum D) minimum

42) Bottom surfaces of kettles are A) blackened B) polished white C) neither blackened nor polished white D) left as such.

43) Velocity of heat radiation v as related to the velocity of light c is

A) v > c B) v = c C) v < c D) no definite relation

44) The wavelength of thermal radiation is

A) less than that of the visible light B) greater than that of the visible light C) equal to that of the visible light D) far shorter than that of the visible light

45) Thermal radiations are similar to

A) X-rays B) cathode rays C) - rays D) sound waves

46) The coefficient of reflectivity of a perfectly black body is

A) zero B) infinity C) unity D) 0.5

47) If the temperature of a black body is raised by 5%, the heat energy radiated would increase by A) 5% B) 20% C) 21.51% D) 41.61%

48) A sphere, a cube and a thin circular plate of same material and mass are heated to

200 0 C. Which one will cool slowest under similar conditions? A) Sphere B) Cube C) Thin plate D) All cool at same rate

49) A black body at a hot temperature of 227 0 C radiates heat at a rate of 10 cal

cm 2 s 1 . At a temperature of 727 0 C the rate of heat radiated per unit area in cal cm 2 s 1 will be A) 200 B) 160 C) 250 D) 400

50) A body takes 4 minutes to cool from 100 0 C to 70 0 C. If the room temperature is

15 0 C, the time taken to cool from 70 0 C to 40 0 C will be A) 7 minutes B) 10 minutes C) 4 minutes D) 2 minutes

51) A body cools from 70 0 C to 50 0 C in 6 minutes. If temperature of surroundings is

30 0 C, what is the temperature after 12 minutes? A) 20 0 C B) 25 0 C C) 35 0 C D) 40 0 C

52) A black body at a high temperature T K radiates energy at the rate E W / m 2 .

When the temperature falls to

2T K, the radiated energy will be

A) E / 4 B) E / 8 C) 2 E D) E / 16

53) A thermos flask is polished well A) to make it attractive B) to make it shine C) to absorb all radiations from outside D) to reflect all radiations from outside

54) A perfect black body emits radiation at temperature T1K. If it is to radiate 16

times this power, its temperature T 2 will be A) T 2 = 16 T 1 B) T 2 = 8 T 1 C) T 2 = 4 T 1 D) T 2 = 2 T 1

55) Liquid is filled in a vessel which is kept in a room at temperature 20 0 C. When the

temperature of the liquid is 80 0 C, it loses heat at the rate of 60 cal / second. The rate of loss of heat when the temperature of the liquid is 40 0 C is A) 180 cal / s B) 40 cal / s C) 50 cal / s D) 20 cal / s

56) In which process is the rate of transfer of heat maximum?

A) Conduction B) Convection C) Radiation D) In all these, heat is transferred with the same speed.

57) Heat is flowing through two cylindrical rods of the same material. The diameters

of the rods are in the ratio 1 : 2 and the lengths in the ratio 2 : 1. If the temperature difference between the ends be the same, then the ratio of the rate of flow of heat through them will be A) 8 : 1 B) 1 : 8 C) 1 : 1 D) 4 : 1

58) A block of steel heated to 100 0 C is left to cool in a room. Which of the following

curves represents the correct behaviour?

A) Curve A B) Curve B C) Curve C D) None of these

59) If the temperature of the hot body is increased by 50%, the amount of radiation

emitted is increased by nearly A) 50% B) 225% C) 250% D) 400%

60) If the temperature of the sun is doubled then

A) emission of energy will be doubled B) emission of energy will become four times C) mostly ultra-violet radiations will be emitted. D) mostly infra-red radiations will be emitted

61) A black body is at a temperature of 500 K. It emits energy at the rate which is

proportional to A) 500 B) (500) 2 C) (500) 3 D) (500) 4

62) A container contains hot water at 100 0 C. If in time T 1 temperature falls to 80

0 C and in time T 2 temperature falls 60

0 C from 80 0 C, then A) T 1 = T 2 B) T 1 > T 2 C) T 1 < T 2 D) None of the above

63) The ratio of the energies of emitted radiation of a black body at 27 0 C and 927 0 C is A) 1 : 4 B) 1 : 16 C) 1 : 64 D) 1 :256

64) How many watts of energy are required to keep a black body in the form of a

cube of side 1 cm at 2000 K? (Temperature of surrounding is 27 0 C and = 5.67 10 8 W m 2 K 4 ) A) 444 W B) 544 W C) 844 W D) 1044 W

65) Due to fall in main voltage, the temperature of the filament of a bulb falls from

3000 K to 2000 K. What is the percentage fall in power consumption? A) 75.25 B) 80.25 C) 85.25 D) 90.25

66) The temperature at which a black body radiates heat at the rate of 5.67 W cm 2 is

( =5.67 10 8 S.I.unit) A) 100 K B) 500 K C) 1000 K D) 1500 K.

67) An electric bulb of tungsten filament has area 0.29 cm 2 and is burning at 2727 0 C.

What is the electric power being consumed, if emissivity is 0.45 and = 5.68 10 8 W m 2 K 4 ? A) 40 W B) 25 W C) 60 W D) 200 W

68) The radiations from the sun have maximum wavelength of 57600A . The surface

temperature of the sun is (b = 0.288 10 2 m K) A) 6000K B) 5000 K C) 4000 K D) 3000 K

69) Fraunhoffers lines in the solar spectrum are in accordance with

A) Kirchhoffs law B) Wiens displacement law C) Newtons law of cooling D) Stefans law

70) If wavelength of maximum intensity of radiations emitted by sun and moon are

0.5 10 6 m and 10 4 m respectively, the ratio of their temperature is A) 1 / 200 B) 1 / 100 C) 100 D) 200

71) A polished metal plate with a rough black spot on it, is heated to about 1400 K

and quickly taken into a dark room. Which of the following statements will be true? A) The spot will appear brighter than the plate B) The spot will appear darker than the plate C) The spot and the plate will appear equally bright D) The spot and the plate will not be visible in the dark room

72) An object is at a temperature of 400 0 C. At what temperature would it radiate

energy twice as fast? The temperature of the surroundings may be assumed to be negligible A) 200 0 C B) 200 0 K C) 800 0 C D) 800 0 K

73) The temperature of a furnace is 2324 0 C and the intensity is maximum in its

radiation spectrum at 12 10 30A . If the intensity in the spectrum of a star is

maximum at 48000A , what is the surface temperature of the star?

A) 621.95 o C B) 6219.5 0 C C) 62195 0 C D) None of the above

74) The filament of a bulb has an area 5 10 5 m 2 and is at 2000K. If its relative emittance is 0.85 and = 5.7 10 8 MKS units, what is the energy radiated in one minute. A) 2326 J B) 232.6 J C) 23.26 J D) None of the above

75) Energy is being emitted from the surface of a black body at 127 0 C temperature at

1 10 6 J s 1 m 2 . Temperature of the black body at which the rate of emission is 16 10 6 J s 1 m 2 will be A) 254 0 C B) 508 0 C C) 527 0 C D) 272 0 C

76) Solar radiation emitted by the sun resembles that emitted by a black body at 6000

K. Maximum intensity is emitted at wavelength 4800 0A . If the sun were to be

cooled down from 6000 K to 3000 K, then peak intensity would occur at a wavelength of

A) 4800 0A

B) 9600 0A

C) 2400 0A

D) 19200 0A

77) Certain substances emit only the wavelengths 1 , 2 , 3 and 4 when it is at

high temperature. When this substance is at a low temperature, it will absorb only the following wavelength/s A) 1 B) 2 C) 21, D) 4321 ,,,

78) In an atomic bomb, a temperature of 10 million degrees in developed at the

moment of explosion. In what region of the spectrum does the wavelength corresponding to maximum energy density lie?

(b=0.28 10 2 S.I. unit) A) ultra-violet B) visible C) infra-red rays D) x-rays

79) A body in a room cools from 85 0 C to 80 0 C in 5 minutes. The time taken to cool

from 80 0 C to 75 0 C is A) 5 minutes B) less than 5 minutes C) more than 5 minutes D) may be less than or more than 5 minutes

80) Two blocks of ice when pressed together join to form one block because

A) of heat produced during pressing B) of cold produced during pressing C) melting point of ice decreases with increase of pressure D) melting point of ice increases with increase in pressure

81) At 0 0 C a body emits

A) no radiation B) electromagnetic radiation of single wavelength C) electromagnetic radiation of all wavelengths that are emitted by it at room

temperature D) Electromagnetic radiation of fewer wavelengths than that emitted by it at

room temperature.

82) Weins displacement law fails for A) short wavelengths B) long wavelengths C) low temperature D) high temperature

83) The wavelength of maximum emission for moon is 14 micron. Estimate the temperature of moon if b = 28.84 10 4 mK is

A) 150 K B) 206 K C) 306 K D) 500 K

84) The temperature of a furnace is 2324 C and the intensity is maximum in its

radiation spectrum nearly at 1200 0A . If the intensity in the spectrum of star is

maximum nearly at 4800 0A , then the surface temperature of the star is

A) 4800 C B) 7200 C C) 6500 C D) 5900 C

85) A black body at a high temperature T radiates energy at the rate of E (in W m-2).

When the temperature falls to 2T , the radiated energy ( in W m-2) will be

A) 4E

B) 2E

C) 2E

D) 16E

86) The running of fan makes us comfortable during summer, because it

A) decreases the temperature of air B) increases the thermal conductivity of air C) increases the rate of evaporation of perspiration D) cuts off the thermal radiation reaching us

87) Two spheres of 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 A) greater that that by the second B) less than that by the second C) equal in both cases D) the information is incomplete to draw any conclusion.

88) Melting point of ice

A) is independent of pressure B) decreases with pressure C) increases with pressure D) is proportional to pressure

89) A large iceberg melts at the base but not at the top because

A) the base of iceberg remains in warmer conditions B) ice at the top contains impurities C) due to high pressure ice at the base lowers its melting point D) ice at the top is different

90) When water is heated from 0 C to 10 C then its volume

A) increases B) decreases C) first increases and then decreases D) first decreases and then increases

91) The absolute temperature of a perfectly black body is doubled. The heat radiated from it will increase how many times?

A) 4 times B) 8 times C) 16 times D) 32 times

92) Two bodies are at temperatures 27 C and 327 C. The heat energy radiated by them will be in the ratio

A) B) 1/8 C) 1/16 D) (27/327)4

93) A metal ball of surface area 200 cm2 , temperature 527 C is surrounded by a vessel at 27 C. If the emissivity of metal is 0.4, then the rate of loss of heat is

nearly ( = 5.67 10-8 Wm-1 k-4) A) 108W B) 168 W C) 182 W D) 192W

94) A cube and a sphere of the same material have equal surface areas. Both are at

the same temperature 120 C. Then A) both the cube and the sphere cool at the same rate B) the cube cools faster than the sphere C) the sphere cools faster than the cube D) whichever is heavier will cool faster.

95) The earth receives at its surface radiation from the sun at the rate of 1400 W/ m2 .

The distance of the centre of the sun from the surface of the earth is 1.5 x 1011 m and the radius of the sun is 7.0 108 m. Treating the sun as a black body, it follows from the above data that its surface temperature is A) 5801 K B) 106 K C) 50.1 K D) 5801 C

6. HEAT RADIATION

Ques. No. Answer 1 D 2 C 3 D 4 C 5 C 6 B 7 C 8 D 9 B

10 D 11 D 12 D 13 D 14 D 15 B 16 C 17 C 18 A 19 C 20 D 21 A 22 D 23 D 24 D 25 A 26 D 27 D 28 C 29 C 30 C 31 B 32 B 33 A 34 B 35 C 36 A 37 D 38 D 39 B 40 B 41 C 42 A 43 B

Ques. No. Answer 44 B 45 A 46 A 47 C 48 A 49 B 50 A 51 C 52 D 53 D 54 D 55 D 56 C 57 B 58 A 59 D 60 C 61 D 62 C 63 D 64 B 65 B 66 C 67 C 68 B 69 A 70 D 71 A 72 D 73 B 74 A 75 C 76 B 77 D 78 D 79 C 80 C 81 C 82 B 83 B 84 C 85 D 86 C 87 C

Ques. No. Answer 88 B 89 C 90 D 91 C 92 C 93 C 94 C 95 A