IISER Physics - Transference of Heat
Exam Duration: 45 Mins Total Questions : 30
A body at \(0ºC\)
- (a)
does not emit any radiation
- (b)
emits monochromatic radiation
- (c)
emits radiation of all those wavelengths which are emitted by it at room temperature
- (d)
emits fewer wavelengths than it would at room temperature
The maximum wavelength of radiation emitted at 200 K is 4 \(\mu m\). What will be maximum wavelength emittted at 2400 K?
- (a)
3.3 \(\mu m\)
- (b)
0.66 \(\mu m\)
- (c)
1 metre
- (d)
1 \(\mu m\)
Two vessels of different materials are similar in size in every respect. The same quantity of ice filled in them gets melted in 20 min and 40 min respectively. The ratio of thermal conductivities of the metals is
- (a)
5 : 6
- (b)
6 : 5
- (c)
2 : 1
- (d)
1 :3
The ratio of thermal conductivities of two rods is 5 :2. If the thermal resistance of the two rods of same thickness of these materials is same, then the ratio of lengths of the rods is
- (a)
5/2
- (b)
2/5
- (c)
1
- (d)
5/7
Two rods of length d1 and d2 and and coefficients of thermal conductivities K1 and K2 are kept touching each other. Both have same area of cross-section; the equivalent thermal conductivity is
- (a)
\({ K }_{ 1 }+{ K }_{ 2 }\)
- (b)
\({ K }_{ 1 }{ d }_{ 1 }+{ K }_{ 2 }{ d }_{ 2 }\)
- (c)
\(\frac { { d }_{ 1 }{ K }_{ 1 }+{ d }_{ 2 }{ K }_{ 2 } }{ { d }_{ 1 }+{ d }_{ 2 } } \)
- (d)
\(\frac { { d }_{ 1 }+{ d }_{ 2 } }{ \frac { { d }_{ 1 } }{ { K }_{ 1 } } +{ \frac { { d }_{ 2 } }{ { K }_{ 2 } } } } \quad \)
Which of the following qualities are best suited for a cooking utensil?
- (a)
High specific heat and low thermal conductivity
- (b)
High specific heat and high thermal conductivity
- (c)
Low specific heat and low thermal conductivity
- (d)
Low specific heat and high thermal conductivity
The presence of gravitational field is required for the heat transfer by
- (a)
stirring of liquids
- (b)
conduction
- (c)
natural convection
- (d)
radiation
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 7.0x108 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)
5801oC
Following the laws of radiation, a cadmium salt when put into a buns en flame gives red colour to the flame. If white light from a tungsten filament lamp is made to pass through a bulb containing cadmium vapours, the transmitted light will be:
- (a)
red
- (b)
blue
- (c)
white
- (d)
white light without red part
Two rods of equal length and diameter have thermal conductivities 3 and 4 units respectively. If they are joined in series, the thermal conductivity of the combination would be:
- (a)
3.43
- (b)
3.5
- (c)
3.4
- (d)
3.34
Under steady state the temperature of a body:
- (a)
increases with time
- (b)
decreases with time
- (c)
does not change with time and is same at all points of the body
- (d)
does not change with time but is different at different cross-sections of the body
One feels hotter at the top of a flame than the sides because of:
- (a)
conduction
- (b)
convention
- (c)
radiations
- (d)
both (a) and (c)
Two spheres of the same material have radii 1 m and 4 m and temperatures 4000 K and 2000 K respectively. The energy radiated per second by the first sphere is:
- (a)
greater than 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
Solar radiation emitted by the 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. If the sun was cooled down from 6000 K to 3000 K, then the peak intensity would occur at a wavelength of:
- (a)
4800\(\mathring { A } \)
- (b)
9600\(\mathring { A } \)
- (c)
2400\(\mathring { A } \)
- (d)
19200\(\mathring { A } \)
The coefficients ofthermal conductivity of copper, mercury and glass are respectively Kc ,Km and Kg such that Kc > Km> Kg. If the same quantity of heat is to flow per second per unit area of each and corresponding temperature gradients are Xc, Xm and Xg:
- (a)
Xc=Xm=Xg
- (b)
Xc>Xm>Xg
- (c)
Xc<Xm<Xg
- (d)
Xm<Xc<Xg
Two rods of copper and brass having the same length and cross-section are joined end to end. The free end of the copper rod is at ooe and of the brass rod is at 100°C in steady state. If thermal conductivity of copper is 4 times of that of brass, find the temperature at the junction of two rods.
- (a)
800C
- (b)
600C
- (c)
400C
- (d)
200C
If the temperature of the sun was to increase from T to 2T and its radius from R to 2R, then the ratio of the radiant energy received on the earth to what it was previously will be:
- (a)
4
- (b)
16
- (c)
32
- (d)
64
Two rods of equal length and area of cross-section are kept parallel and lagged between temperatures 20°C and 80°C. The ratio of the effective thermal conductivity to that of the first rod is: [the ratio (K1/K2) = 3:4]
- (a)
7 : 4
- (b)
7 : 6
- (c)
4 : 7
- (d)
7 : 8
Two cylinders P and Q have the same length and diameter, and are made of different materials having thermal conductivities in the ratio 2 : 3. These two cylinders are combined to make a cylinder. One end of P is kept at 100°C and the other end of Q at O°C. The temperature at the interface of P and Q is:
- (a)
30°C
- (b)
40°C
- (c)
50°C
- (d)
60°C
In winter, an iron chair appears colder than a wooden chair. This is due to:
- (a)
conduction
- (b)
convection
- (c)
radiation
- (d)
boiling
Thermal radiations are electromagnetic waves belonging to:
- (a)
ultraviolet region
- (b)
visible region
- (c)
gamma region
- (d)
infrared region
The absolute temperature of a body A is four times that of another body B. For two bodies, the difference ill wavelengths at which energy radiated is maximum, is 3.0 μm. Then, the wavelength at which the body B radiates maximum energy (in micro metre) is:
- (a)
2
- (b)
2.5
- (c)
4.0
- (d)
4.5
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 \(\left( \frac { { T }_{ 2 } }{ { T }_{ 1 } } \right) \)
An electric heater is placed inside a room of total wall area 137 m2 and maintained at a temperature 20°C inside, outside temperature -10°C. The walls are made up of three composite materials. Innermost layer is made up of wood of thickness 2.5 cm, middle layer is of cement of thickness 1em and the exterior layer is 25 cm thick. Assuming there is no loss of heat through any other way, the power of electric heater is: [the thermal conductivity of wood = 0.125 W/m2 oC, cement 1.5 W/m2°C and brick = 1W/m2 0C]
- (a)
9000 W
- (b)
8500 W
- (c)
8800 W
- (d)
9400 W
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
If the amount of heat energy received per unit area from the sun is measured on the earth, mars and jupiter, it will be:
- (a)
same for all
- (b)
in decreasing order jupiter, mars and the earth
- (c)
in increasing order jupiter, mars and the earth
- (d)
in decreasing order mars, the earth and jupiter
A body of area 1cm2 is heated to a temperature 1000 K. The amount of energy radiated by the body in 1s is (Stefan's constant σ = 5.67 x 10-8 Wm-2K-4) :
- (a)
5.67 joule
- (b)
0.567 joule
- (c)
56.7 joule
- (d)
567 joule
A liquid in a beaker has temperature θ(t) at time t and θ0 is temperature of surroundings, then according to Newton's law of cooling the correct graph between loge(θ - θ0) and t is:
- (a)
- (b)
- (c)
- (d)
Three very large plates of same area are kept parallel and close to each other. They are considered as ideal black surfaces and have very high thermal conductivity. The first and third plates are maintained at temperature 2T and 3T respectively. The temperature of the middle (i.e., second) plate under steady state condition is:
- (a)
\({ \left( \frac { 65 }{ 2 } \right) }^{ 1/4 }T\)
- (b)
\({ \left( \frac { 97 }{ 4 } \right) }^{ 1/4 }T\)
- (c)
\({ \left( \frac { 97 }{ 2 } \right) }^{ 1/4 }T\)
- (d)
(97)1/4 T
If a piece of metal is heated to temperature 8 and then allowed to cool in a room which is at temperature 80, the graph between the temperature T of the metal and time (will be closest to :
- (a)
- (b)
- (c)
- (d)