A human body has a surface area of approximately 1 m 2 . The normal body temperature is 10 K above the surrounding room temperature T 0 . Take the room temperature to be T 0 = 300 K . For T 0 = 300 K , the value of σ T 4 = 460 W m - 2 (where σ is the Stefan-Boltzmann constant). Which of the following options is/are correct?

Reducing the exposed surface area of the body (e.g. by curling up) allows humans to maintain the same body temperature while reducing the energy lost by radiation.

If the surrounding temperature reduces by a small amount Δ T 0 < < T 0 , then to maintain the same body temperature the same (living) human being needs to radiate Δ W = 4 σ T 0 3 Δ T 0 more energy per unit time

If the body temperature rises significantly then the peak in the spectrum of electromagnetic radiation emitted by the body would shift to longer wavelengths

The amount of energy radiated by the body in 1 second is close to 60 J o u l e s

An incandescent bulb has a thin filament of tungsten that is heated to high temperature by passing an electric current. The hot filament emits black-body radiation. The filament is observed to break up at random locations after a sufficiently long time of operation due to non-uniform evaporation of tungsten from the filament. If the bulb is powered at constant voltage, which of the following statement(s) is(are) true?

The filament consumes less electrical power towards the end of the life of the bulb

The temperature distribution over the filament is uniform

The resistance over small sections of the filament decreases with time

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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 difference between each sphere and its surroundings is $T$ . Then,

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Important Questions on Transmission of Heat

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 16

A block of metal is heated to a temperature much higher than the room temperature and allowed to cool in a room free from air currents. Which of the following curves correctly represents the rate of cooling?

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 17

Heat is flowing through a conductor of length

l

from

x = 0

x = l

, If its thermal resistance per unit length is uniform, which of the following graphs is correct?

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 18

Two rectangular blocks

A

and

B

of different metals have same length and same area of cross-section. They are kept in such a way that their cross-sections touch each other. The temperature at one end of

A

100 ° C

and that of

B

at the other end is

0 ° C

. If the ratio of their thermal conductivities is

1 : 3

, then under steady state, the temperature of the junction in contact will be

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 19

Two rods of 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 along their lengths, then they will transfer same heat in same conditions in

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 20

The coefficients of thermal conductivity of copper, mercury and glass are respectively

K_{c}, K_{m}

and

K_{g}

such that

K_{c} > K_{m} > K_{g}

. If the same quantity of heat is to flow per second per unit area of each and corresponding temperature gradients are

X_{c}, X_{m}

and

X_{g}

, then

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 22

Two spheres of different materials one with double the radius and one-fourth wall thickness of the other, are filled with ice. If the time taken for melting complete ice in the larger sphere is

25

minutes and that for smaller one is

16

minutes, the ratio of thermal conductivity of the material of larger sphere to that of the smaller sphere is

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 23

A metal rod of length 2m has cross sectional areas 2A and A as shown in figure. The ends are maintained at temperatures 100°C and 70°C. The temperature at middle point C is
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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 24

In a steady state of thermal conduction, temperature of the ends

A

and

B

of a

20 cm

long rod are

100 ° C

and

0 ° C

, respectively. What will be the temperature of the rod at a point at a distance of

6 cm

from the end

A

of the rod?

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 difference between each sphere and its surroundings is T. Then,

Important Questions on Transmission of Heat

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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 difference between each sphere and its surroundings is $T$ . Then,