The absorptance is defined as the ratio of the radiation (power) absorbed within a given waveband λ to λ + d λ to the radiation (power) incident on the body in the same waveband. Mathematically, a λ = P λ a P λ i = P λ a d t P λ i d t a λ = E λ a E λ i

MEDIUM

Earn 100

What do you understand by absolute emissivity.

Important Questions on Heat Transfer

HARD

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Three large identical plates are kept parallel to each other. The outer two plates are maintained at temperatures

T

and

2 T

, respectively. The temperature of the middle plate in steady state will be close to

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

The wavelength of thermal radiation is

MEDIUM

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Heat energy is incident on the surface of a material at the rate of

1000 J \min^{- 1}

. If the coefficient of absorption is

0.8

and the coefficient of reflection is

0.1

, then the amount of heat energy transmitted through the material in

5 minutes

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Let there be four identical cubes having colours blue, red, black and white. When they are heated together and allowed to cool under identical conditions, which cube will cool at the earliest?

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

For any material, if

R, T

and

A

represent the reflection coefficient, transparent coefficient and absorption coefficient respectively, then for a blackbody which one of the following is true

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

(A)	The energy emitted versus wavelength plot of a black body depends on the material of the black body.
(B)	The emissivity of a completely reflecting surface is zero.
(C)	Emissivity is equal to ${(absorptive power)}^{2}$ .
(D)	Black-body radiates more than normal body Which of the following is correct?

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

A certain planet is at

a

distance

d

from the sun. Then the temperature of the planet is

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Prevost's theory of heat exchange is not applicable at temperature

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

If the surface is a perfect reflector, the change in momentum of the wave after falling on the surface is

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Define spectral radiant existence.

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Assertion: Emissive power of a perfectly black body is one.

Reason: Absorptive power of a perfectly black body is one.

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Define absorptance.

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Explain why the bottom of cooking vessel is made black?

HARD

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Assuming that your surface temperature is

98.6 ° F

and that you are an ideal blackbody radiator (you are close), find

(a)

the wavelength at which your spectral radiancy is maximum,

(b)

the power at which you emit thermal radiation in a wavelength range of

1.00 nm

at that wavelength, from a surface area of

4.00 {cm}^{2},

and

(c)

the corresponding rate at which you emit photons from that area. Using a wavelength of

500 nm

(in the visible range),

(d)

recalculate the power and

(e)

the rate of photon emission. (As you have noticed, you do not visibly glow in the dark).

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Define total radiant exitance.

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

The spectrum of black body radiation is a

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Define total emissivity.

HARD

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

Solar constant is 1370 W/m². 70% of the light incident on the earth is absorbed by the earth and the earth's average temperature is 288 K. The effective emissivity of the earth is

EASY

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

The area enclosed between $E_{\lambda}-\lambda$ curve and $\lambda-$ axis is equal to

HARD

Physics>Heat and Thermodynamics>Heat Transfer>Radiation

The emissive power of a black body of surface area

A = 10 m^{2}

T = 300 K

10

watt

/ m^{2} .

Consider a body

B

of area

A = 10 m^{2},

coefficient of reflectivity

r = 0.3

and coefficient of transmission

t = 0.5 .

Its temperature is

300 K .

Then emissive power of

B

W / m^{2}

is.......

What do you understand by absolute emissivity.

Important Questions on Heat Transfer

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