51. A black body at $${227^ \circ }C$$ radiates heat at the rate of $$7cal\,c{m^{ - 2}}{s^{ - 1}}.$$ At a temperature of $${727^ \circ }C,$$ the rate of heat radiated in the same units will be
A
60
B
50
C
112
D
80
Answer :
112
52. A kettle with 3 litre water at $${27^ \circ }C$$ is heated by operating coil heater of power $$2\,kW.$$ The heat is lost to the atmosphere at constant rate $$130\,J/\sec,$$ when its lid is open. In how much time will water heated to $${97^ \circ }C$$ with the lid open? (specific heat of water $$= 4.2\,kJ/kg$$ )
A
$$472\,\sec $$
B
$$693\,\sec $$
C
$$912\,\sec $$
D
$$1101\,\sec $$
Answer :
$$472\,\sec $$
53.
A body cools in a surrounding which is at a constant temperature of $${\theta _0}.$$ Assume that it obeys Newton’s law of cooling. Its temperature $$\theta $$ is plotted against time $$t.$$ Tangents are drawn to the curve at the points $$P\left( {\theta = {\theta _2}} \right)$$ and $$Q\left( {\theta = {\theta _1}} \right).$$ These tangents meet the time axis at angle of $${\phi _2}$$ and $${\phi _1},$$ as shown, then
A
$$\frac{{\tan {\phi _2}}}{{\tan {\phi _1}}} = \frac{{{\theta _1} - {\theta _0}}}{{{\theta _2} - {\theta _0}}}$$
B
$$\frac{{\tan {\phi _2}}}{{\tan {\phi _1}}} = \frac{{{\theta _2} - {\theta _0}}}{{{\theta _1} - {\theta _0}}}$$
C
$$\frac{{\tan {\phi _1}}}{{\tan {\phi _2}}} = \frac{{{\theta _1}}}{{{\theta _2}}}$$
D
$$\frac{{\tan {\phi _1}}}{{\tan {\phi _2}}} = \frac{{{\theta _2}}}{{{\theta _1}}}$$
Answer :
$$\frac{{\tan {\phi _2}}}{{\tan {\phi _1}}} = \frac{{{\theta _2} - {\theta _0}}}{{{\theta _1} - {\theta _0}}}$$
54. Infrared radiation is detected by
A
spectrometer
B
pyrometer
C
nanometer
D
photometer
Answer :
pyrometer
55. The wavelength corresponding to maximum intensity of radiation emitted by a source at temperature $$2000\,K$$ is $$\lambda ,$$ then what is the wavelength corresponding to maximum intensity of radiation at temperature $$3000\,K$$ ?
A
$$\frac{2}{3}\lambda $$
B
$$\frac{{16}}{{81}}\lambda $$
C
$$\frac{{81}}{{16}}\lambda $$
D
$$\frac{4}{3}\lambda $$
Answer :
$$\frac{2}{3}\lambda $$
56. If a piece of metal is heated to temperature $${\theta}$$ and then allowed to cool in a room which is at temperature $${\theta _0},$$ the graph between the temperature $$T$$ of the metal and time $$t$$ will be closest to
A
B
C
D
Answer :
57. An ideal Black-body at room temperature is thrown into a furnace. It is observed that
A
initially it is the darkest body and at later times the brightest
B
it is the darkest body at all times
C
it cannot be distinguished at all times
D
initially it is the darkest body and at later
Answer :
initially it is the darkest body and at later times the brightest