Tag: physics

Questions Related to physics

Ferry's black body is accurately represented by 

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

  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.

Show answer
Correct Option: B
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. 

Ferry's black body is 

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

  1. a perfect reflector of heat.

  2. perfectly black.

  3. platinum black.

  4. almost perfectly black.

Show answer
Correct Option: D
Explanation:
Ferry's black body is almost perfectly black.
As because emission of heat and absroption of heat takes place completely.

Black body radiation is

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

  1. White

  2. IR

  3. Black

  4. UV

Show answer
Correct Option: A
Explanation:

a black body looks black because it absorbs light of all colors. so when it radiates energy as light, it will release all the colors which it had absorbed resulting in the emission of white light, because white light is the resultant of many component colors.

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

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  1. $971K$

  2. $1189K$

  3. $2001K$

  4. $1458K$

Show answer
Correct Option: B
Explanation:

$Rediant Energy = \sigma T^2$

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

Temp. of black body is $3000K$ when black body cools. Then change in wavelength $\Delta \lambda=9$ micron corresponding to maximum energy density. Now temp. of black body is:

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

  1. $300K$

  2. $2700K$

  3. $270K$

  4. $1800K$

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Correct Option: A

The rate of emission of radiation of a black body at 273$^{ \circ  }{ C }$ is E, then the rate of emission of radiation  of this body at 0$^{ \circ  }{ C }$ will be :-

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

  1. $\dfrac { E }{ 16 } $

  2. $\dfrac { E }{ 4 } $

  3. $\dfrac { E }{ 8 } $

  4. 0

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Correct Option: A

For non black bodies, the range of values of emissivity $e$ is 

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

  1. $- 1 < e < 1 $

  2. $- 1 < e < 0 $

  3. $ 1 < e < 2 $

  4. $0 < e < 1 $

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Correct Option: D

A blackbody does not

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

  1. emit radiation

  2. absorb radiation

  3. reflect radiation

  4. refract radiation

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Correct Option: B

An ideal black body is a :

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

  1. lump of charcoal heated to a high temperature

  2. metal coated with a black dye

  3. glass surface coated with coal tar

  4. hollow enclosure blackened inside and having a small hole

Show answer
Correct Option: D
Explanation:

Hollow enclosure blackened inside and having a small hole is a very good example of a black body.
Suppose once light enters inside it.
It may be absorbed or it may be reflected.
Since it is blackened from inside, there is a high probability that it will be absorbed.
Now if it is reflected, it will suffer multiple reflections and it is very unlikely that it will come out of the hole because the aperture of hole is too small. Moreover, with each reflection, more and more fraction of it will be absorbed. So, it will serve as a good black body.

Which of the following is more close to a black body?

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

  1. Black board paint

  2. Green leaves

  3. Black holes

  4. Red roses

Show answer
Correct Option: C
Explanation:

we know that $\alpha +\rho +\tau =1$
${\alpha}= absorptivity$
${\rho}=reflectivity$
${\tau}=transmitivity$
so for black body ${\rho}\  and \ {\tau} \ will\  be \ zero$
so ${\alpha}=1$ so black hole has also ${\alpha}=1$ which is equivalent to black body.

Hence we can consider black holes as black body.