Physics - Transference of Heat
Exam Duration: 45 Mins Total Questions : 30
Thermal radiations are similar to
- (a)
electromagnetic waves
- (b)
cathode rays
- (c)
sound waves
- (d)
alpha rays
An object is cooled from 75°C to 65°C in 2 minutes in a room at 30°C. The time taken to cool the same object from 55°C to 45°C in the same room in minutes is
- (a)
4
- (b)
5
- (c)
6
- (d)
7
A cylinder of radius R made of a material of thermal conductivity K1 is surrounded by a cylindrical shell of inner radius R and outer radius 2R made of a material of thermal conductivity K2. The two ends of the combined system are maintained at two different temperatures. There is no loss of heat across the cylindrical surface and the system is in steady state. The effective thermal conductivity of the system is:
- (a)
K1 + K2
- (b)
\(\frac { { K }_{ 1 }+3{ K }_{ 2 } }{ 4 } \)
- (c)
\(\frac { { K }_{ 1 }{ K }_{ 2 } }{ { K }_{ 1 }+{ K }_{ 2 } } \)
- (d)
\(\frac { 3{ K }_{ 1 }+{ K }_{ 2 } }{ 4 } \\ \)
The wavelength of maximum intensity of emission of solar radiation is \({ \lambda }_{ m }=4753\mathring { A } \) and from the moon it is 14 mm. The surface temperature of the sun and the moon are: (given b = 2.898 \(\times\) 10-3 metre/kelvin)
- (a)
6097 K, 207 K
- (b)
8097 K, 307 K
- (c)
10000 K, 400 K
- (d)
3000 K, 100 K
Two identical rods with different thermal conductivities K1 and K2 and different temperatures are first placed along length and then along area, then the ratio of rates of heat flow in both cases is:
- (a)
\(\frac { 4{ K }_{ 1 }{ K }_{ 2 } }{ { ({ K }_{ 1 }+{ K }_{ 2 } })^{ 2 } } \)
- (b)
\(\frac { { K }_{ 1 } }{ { K }_{ 2 } } \)
- (c)
\(\frac { { K }_{ 1 }+{ K }_{ 2 } }{ { K }_{ 1 }-{ K }_{ 2 } } \)
- (d)
none of these
The temperature of a room heated by a heater is 20°C when outside temperature is -20°C and it is 10°C when the outside temperature is -40°c. The temperature of the heater is:
- (a)
80°C
- (b)
100°C
- (c)
40°C
- (d)
60°C
Let there be four articles having colours blue, red, black and white. When these articles are heated and then allowed to cool, which article would cool at the earliest?
- (a)
Blue
- (b)
Red
- (c)
Black
- (d)
White
A solid sphere and a hollow sphere of same material and size are heated to same temperature and allowed to cool in the same surroundings. If the temperature difference between each sphere and its surroundings 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 coo 1at the same rate for all values of T
- (d)
both spheres will cool at the same rate only for small values of T
When a hot and a cold body are placed In a vacuum container, the hot body cools due to:
- (a)
radiations emitted by the hot body
- (b)
conduction by the hot body
- (c)
radiations emitted by cold body
- (d)
convection in the hot body
Two similar rods but of different materials are joined together end to end. The ratio of their thermal conductivities is 2: 3. The composite rod is lagged between melting ice at 00C and steam at normal pressure. The temperature of the junction will be:
- (a)
600C
- (b)
500C
- (c)
400C
- (d)
300C
In a composite rod, when two rods of different lengths and of the same area of cross-section are joined end to end, then if K is the effective coefficient of thermal conductivity, \(\frac { { l }_{ 1 }+{ l }_{ 2 } }{ K } \) is equal to:
- (a)
\(\frac { { l }_{ 1 } }{ { K }_{ 1 } } -\frac { { l }_{ 2 } }{ { K }_{ 2 } } \)
- (b)
\(\frac { { l }_{ 1 } }{ { K }_{ 2 } } -\frac { { l }_{ 2 } }{ { K }_{ 1 } } \)
- (c)
\(\frac { { l }_{ 1 } }{ { K }_{ 1 } } +\frac { { l }_{ 2 } }{ { K }_{ 2 } } \)
- (d)
\(\frac { { l }_{ 1 } }{ K_{ 2 } } +\frac { { l }_{ 2 } }{ { K }_{ 1 } } \)
A piece of glass is heated to a high temperature and then allowed to cool. If it cracks, a probable reason for this is the following property of glass:
- (a)
low thermal conductivity
- (b)
high thermal conductivity
- (c)
high specific heat
- (d)
high melting point
It is hotter at same distance over the fire than in front of it because:
- (a)
air conducts heat upward only
- (b)
heat is radiated upwards only
- (c)
air is a bad conductor
- (d)
convection of heat occurs upwards only
If a liquid is heated in weightlessness, the heat is transmitted through:
- (a)
conduction
- (b)
convection
- (c)
radiation
- (d)
neither, because the liquid cannot be heated in weightlessness
A wall has two layers A and B, each made of a different material. Both the layers have the same thickness. The thermal conductivity of the material of A is twice that of B. Under thermal equilibrium, the temperature difference across the wall is 36°C. The temperature difference across the layer A is:
- (a)
6°C
- (b)
12oC
- (c)
18°C
- (d)
24°C
The rate of dissipation of heat by a black body at temperature T is Q . What will be the rate of dissipation of heat by another body at temperature 2T and emissivity 0.25?
- (a)
16Q
- (b)
4Q
- (c)
8Q
- (d)
4.5Q
Thermal radiations are electromagnetic waves belonging to:
- (a)
ultraviolet region
- (b)
visible region
- (c)
gamma region
- (d)
infrared region
A planet radiates heat at a rate proportional to the fourth power of its surface temperature T. If such a steady temperature of the planet is due to an exactly equal amount of heat received from the sun then which of the following statement is true?
- (a)
The planet's surface temperature varies inversely as the distance of the sun.
- (b)
The planet's surface temperature varies directly as the square of its distance from the sun.
- (c)
The planet's surface temperature varies inversely as the square root of its distance from the sun.
- (d)
The planet's surface temperature is proportional to the fourth power of its distance from the sun.
A thin wire of length 50 cm and surface area S = 3 x 10-4 m2 is heated to 727°C. How much electric power P is needed to maintain the wire at this temperature? (Emissivity of surface is e = 0.25 , Stefan's constant = 5.67x 10-8Wm-2 K-4)
- (a)
4.2 W
- (b)
2.1 W
- (c)
8.4 W
- (d)
6 W
A rod of length I with thermally insulated lateral surface is made of a material whose thermal conductivity K varies as K = C/T, where C is a constant. The ends are at temperatures T1 and T2. The heat flow density is:
- (a)
Clog \(\frac { { T }_{ 2 } }{ { T }_{ 1 } } \)
- (b)
\(\frac { C }{ l } \log { \left( \frac { { T }_{ 2 } }{ { T }_{ 1 } } \right) } \)
- (c)
\(\frac { C }{ l } \log { \left( { T }_{ 1 }{ T }_{ 2 } \right) } \)
- (d)
Cl log \(\left( \frac { { T }_{ 2 } }{ { T }_{ 1 } } \right) \)
The power of a black body at temperature 200 K is 544 W. Its surface area is: (σ = 5.67 x 10-8 Wm-2K-4)
- (a)
6 x 10-2 m2
- (b)
6 m2
- (c)
6 x 10-6 m2
- (d)
6 x 102 m2
In which of the following heat loss is primarily not due to convection?
- (a)
Boiling of water
- (b)
Land and sea breeze
- (c)
Heating of glass surface of a bulb due to current in filament
- (d)
Circulation of air around blast furnace
Two rods of same material have same length and area. The heat ΔQ flows through them for 12 minutes when they are joint side by side. If now both the rods are joined in parallel, then the same amount of heat ΔQ will flow in:
- (a)
24 min
- (b)
3 min
- (c)
12 min
- (d)
6 min
Two bodies A and B having temperature 327°C and 427°C are radiating heat to the surrounding. The surrounding temperature is 27°C. The ratio ofrate of heat radiation of A to that of B is:
- (a)
0.52
- (b)
0.31
- (c)
0.81
- (d)
0.42
Solar pond is a device for collecting solar heat. The pond is about one metre deep, filled with saturated salt solution and protected from air current and other disturbances. When exposed to the sun, the temperature at the bottom can go as high as 800eor more. Why is this possible?
- (a)
Due to convection, the heated water goes down.
- (b)
Density gradient prevents convection currents.
- (c)
Thermal conductivity of salt is very high.
- (d)
Thermal conductivity of water is very low
The end A of a rod AB oflength 1m is maintained at 1000C and the end B at 100C. The temperature at a distance of 60 cm from the end B is:
- (a)
64°C
- (b)
36°C
- (c)
46°C
- (d)
72°C
In which ofthe following phenomena, the heat waves travel along straight lines with the speed of light?
- (a)
Thermal conduction
- (b)
Forced convection
- (c)
Natural convection
- (d)
Thermal radiation
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