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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The earth receives on its surface radiation from the sun at the rate of $1400 W m^{- 2}$ . The distance of the centre of the sun from the surface of the earth is $1.5 \times 10^{11} m$ and the radius of the sun is $7.0 \times 10^{8} m$ . Treating sun as a block body, it follows from the above data that its surface temperature is

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Important Questions on Transmission of Heat

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Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 60

Three rods of identical cross-sectional area and made from the same metal, form the sides of an isosceles triangle $A B C$ , right angled at $B$ , as shown in the figure. The points $A$ and $B$ are maintained at temperatures $T$ and $\sqrt{2} T$ , respectively, in the steady state. Assuming that only heat conduction takes place, temperature of point $C$ will be

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 61

Two hollow spheres of different materials, one with double the radius and one-fourth wall thickness of the other, are filled with ice. If the times taken for complete melting of ice in the larger to the smaller one are in the ratio of

25 : 16

, then their corresponding thermal conductivities are in the ratio

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 62

A room at

20 ° C

is heated by a heater of resistance

20 ohm

connected to

200 V

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

1 m^{2}

and thickness

0 .2 cm

. Calculate the temperature outside. Thermal conductivity of glass is

0.2 cal / m ° C s

and mechanical equivalent of heat is

4 .2 J/cal

EASY

Physics

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 63

When the temperature of a black body increases, it is observed that the wavelength corresponding to maximum energy changes from

0.26 μm

0.13 μm

. The ratio of emissive powers of the body at the respective temperature is

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Physics

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 64

The radiation emitted by a star

A

is 10,000 times that of the sun. If the surface temperature of the sun and the star

A

are 6000 K and 2000 K respectively, the ratio of the radii of the star

A

and the sun is

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 65

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 statements is true?

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Physics

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PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 66

A black body is at a temperature of

2880 K

. The energy of radiation emitted by this object with wavelength between

499 nm

and

500 nm

U_{1}

, between

999 nm

and

1000 nm

U_{2}

and between

1499 nm

and

1500 nm

U_{3}

. The Wiens constant

b = 2.88 \times 10^{6} nm K

. Then

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Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 2 - Transmission of Heat>Exercises>Q 67

A closed cubical box is made of a perfectly insulating material walls of thickness

8 cm

and the only way for heat to enter or leave the box is through two solid metallic cylindrical plugs, each of cross-sectional area

12 {cm}^{2}

and length

8 cm

, fixed in the opposite walls of the box. The outer surface

A

on one plug is maintained at

100 ° C

, while the outer surface of

B

of the plug is maintained at

4 ° C

. The thermal conductivity of the material of the plug is

0.5 cal / {}^{\circ}C / cm

. A source of energy generating

36 cal / s

is enclosed inside the box. Assuming the temperature to be the same at all points on the inner surface, the equilibrium temperature of the inner surface of the box is

Important Questions on Transmission of Heat

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