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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Six identical conducting rods are joined as shown in the diagram. The points $A$ and $D$ are maintained at temperatures $200 ° C$ and $20 ° C$ , respectively. The temperature of junction $B$ will be,

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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 29

A composite metallic bar of uniform cross-section is made up of length $25 cm$ of copper, $10 cm$ of nickel and $15 cm$ of aluminium, each part being in perfect thermal contact with the adjoining part. The copper end of the composite rod is maintained at $100 ° C$ and the aluminium end at $0 ° C$ . The whole rod is covered with belt so that there is no heat loss at the sides. If $K_{Cu} = 2 K_{Al}$ and $K_{Al} = 3 K_{Ni}$ , then what will be the temperatures of $Cu - Ni$ and $Ni - Al$ junctions, respectively?

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 30

A room is maintained at

20 ° C

by a heater of resistance

20

ohm connected to

200

volt mains. The temperature is uniform throughout the room and heat is transmitted through a glass window of area

1 m^{2}

and thickness

0.2 cm

. What will be the temperature outside? Given that thermal conductivity

K

for glass is

0.2 cal / m / {}^{\circ}C / \sec

and

J = 42 J / cal

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 31

A solid cube and a solid sphere of the same material have equal surface area. Both are at the same temperature

120 ° C

. Then,

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 32

A black metal foil is warmed by radiation from a small sphere at temperature

T

and at a distance

d .

It is found that the power received by the foil is

P

. If both the temperature and the distance are doubled, the power received by the foil will be

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 33

There is a formation of

x cm

thick layer of snow on water, when the temperature of air is

- θ ° C

(less than freezing point). If the thickness of layer increases from

x

y

in the time

t

, then the value of

t

is given by

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 34

The total energy radiated from a black body source is collected for one minute and is used to heat a quantity of water. The temperature of the water is found to increase from

20 ° C

20.5 ° C

. If the absolute temperature of the black body is doubled and the experiment is repeated with the same quantity of water at

20 ° C

, then the temperature of water will be

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 35

A solid copper cube of edges

1 cm

is suspended in an evacuated enclosure. Its temperature is found to fall from

100 ° C

99 ° C

100 s

. Another solid copper cube of edges

2 cm

, with similar surface nature, is suspended in a similar manner. The time required for this cube to cool from

100 ° C

99 ° C

will be approximately.

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 36

A cylindrical rod with one end in a steam chamber and the other end in ice, results in melting of

0.1 g

of ice per second. If the rod is replaced by another, with half the length and double the radius of the first, and if the thermal conductivity of the material of the second rod is

\frac{1}{4}

that of the first, the rate at which ice melts, in

g s^{- 1}

, will be

Six identical conducting rods are joined as shown in the diagram. The points A and D are maintained at temperatures 200°C and 20°C, respectively. The temperature of junction B will be,

Important Questions on Transmission of Heat

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Six identical conducting rods are joined as shown in the diagram. The points $A$ and $D$ are maintained at temperatures $200 ° C$ and $20 ° C$ , respectively. The temperature of junction $B$ will be,