Tag: perfectly black body

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Multiple choice physics energy production perfectly black body black-body radiation black body radiation

Out of the following, which body is not an ideal black body?

  1. Wein's black body

  2. Ferry's black body

  3. coal

  4. sun
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

wein's and ferry's black bodies are well defined in there papers and sun can also be considered as black body because it is very good emitter. but coal is just a normal carbon compound and neither is it a good absorber or emmiter

Multiple choice physics energy production perfectly black body black-body radiation black body radiation

Read the following statements carefully
(A) Black body radiation is white
(B) Emissivity of a body is equal to its absorptive power
Mark correct option:

  1. Statement (A) is correct

  2. Statement (B) is correct

  3. Both are correct

  4. Both are wrong
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
  • (A) The combination of radiation of all the visible wavelengths makes white light. Hence black  body radiation is white

(B) Emissivity of body is equal to it's absorptive power, it is not less then absorptive power or greater than absorptive power
  • Hence both the statements are true
  • Hence option C is the right answer
           

Multiple choice physics energy production perfectly black body black-body radiation black body radiation

Stefan-Boltzmanns Law for a perfect black body is represented by

  1. $\dfrac{dQ}{dt} = \sigma AT^2 $
  2. $\dfrac{dQ}{dt} = \sigma AT^3 $
  3. $\dfrac{dQ}{dt} = \sigma AT^4 $
  4. $ Q = \sigma AT^4 $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Stefan boltzmann's law, $\Rightarrow \cfrac { dQ }{ dt } =\sigma A\varepsilon { T }^{ 4 }$
( for black body, $\varepsilon=1$)
So, for black body stefan's boltzmann's law is $\Rightarrow \cfrac { dQ }{ dt } =\sigma A { T }^{ 4 }$
Stefan's boltzmann's law states that total energy radiated from a surface is proportional to the forth power of its absolute temperature.
Multiple choice physics energy production perfectly black body black-body radiation black body radiation

Ferry's black body is accurately represented by 

  1. A fine hole in a double walled spherical cavity.

  2. A fine hole in a double walled spherical cavity, evacuated and painted black.

  3. A fine hole in a spherical cavity, evacuated and painted black.

  4. A fine hole in a black cavity.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Ferry's black body is accurately represented by a fine hole in a double walled spherical cavity, evacuated and painted black.

$\rightarrow$ Ferry designed the simplest black body. It is a double walled evacuated spherical cavity whose inner wall is blackened. The space between wall is evacuated to prevent heat loss by conduction and radiation. There is a fine hole in it. All the radiations incident upon this hole are absorbed by this black body. 

Multiple choice physics energy production perfectly black body black-body radiation black body radiation

The original temperature of a black body is $727^\circ C$. Calculate temperature at which total radiant energy from this black body becomes double:

  1. $971K$
  2. $1189K$
  3. $2001K$
  4. $1458K$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$Rediant Energy = \sigma T^2$

$Energy = \sigma (1000)^4$
$E _2 = 2 E _1$
$Then$
$\sigma T _2 ^{4} = 2 \times \sigma (1000)^4$
$T _2 = 2^\frac{1}{4} \times1000$
$T _2 = 1189 K$