Tag: heat transfer

Questions Related to heat transfer

If the emissive and absorptive powers of a body are $E$ and $A $ respectively at temperature $T$ then emissive power of a black body will be

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. $E/A$

  2. $EAT$

  3. $EA/T$

  4. $A/E$

Show answer
Correct Option: A
Explanation:

If Emissive power =$ E$
Absorptive power = $A$
The emissive power of a black body is the emissive power for unit absorptive power(per unit absorptive power) (Black body absorbs all radiations)  = $\dfrac{E}{A}$

If $p$ calories of heat energy is incident on a body and $q$ calories is absorbed, then its coefficient of absorption is :

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. $\dfrac{p}{q}$

  2. $p - q$

  3. $\dfrac{q}{p}$

  4. $q + p$

Show answer
Correct Option: C
Explanation:
Calories of incident heat=$p$
Calories of absorbed heat=$q$
$\because$ we know that coefficient of absorption is the ratio of heat absorbed to the incident heat.
So, coefficient of absorption=$\cfrac{q}{p}$

A hot body will emit radiations more rapidly if its surface is

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. black and polished

  2. white and polished

  3. black and rough

  4. white and rough

Show answer
Correct Option: C
Explanation:

$E\propto e \times A$
roughness increases area and black colour increases $e$.

Given that $p$ Joules of heat is incident on a body and out of it $q$ Joules is reflected and transmitted by it, the absorption coefficient of the body is

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. $(q-p)/p$

  2. $q/p$

  3. $(p-q)/p$

  4. $p/q$

Show answer
Correct Option: C
Explanation:

Coefficient of absorption is defined as the ration of amount of radiation a body absorbs to the amount of radiation that is incident on the body.
Incident radiation = $p$
Rejected radiation = $q$
Hence absorbed radiation must be = $p - q$
From the definition of absorptive power it follows that the power = $\dfrac{p-q}{p}$

Kirchoffs law states that

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. a body absorbs radiation of shorter wavelengths and emits radiation of higher wavelength

  2. a body absorbs radiation of any wavelength but emits radiation of specific wavelengths

  3. a body absorbs and emits radiation of same wavelengths

  4. none of these

Show answer
Correct Option: C
Explanation:

In thermodynamics, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium.

If the amount of heat incident upon a body is $X$ calorie and it absorbs $Y$ calorie out of it , then the coefficient of absorption will be

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. $X+Y$

  2. $XY$

  3. $Y/X$

  4. $X/Y$

Show answer
Correct Option: C
Explanation:

Coefficient of absorption is defined as the ration of amount of radiation a body absorbs to the amount of radiation that is incident on the body.
Incident radiation = $X$
Absorbed radiation  = $Y$
Hence absorptive power = $\dfrac{Y}{X}$

Good absorbers of heat are 

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. Poor emitters of heat

  2. Good emitters of heat

  3. Only black bodies

  4. None of these

Show answer
Correct Option: B
Explanation:
Good absorbers of heat are good emitters of heat.
It is better explained by the Kirchoff's law of radiation.
For eg - Black Body is a good emitter as well as good absorber of heat.

The temperature of a room heated by a heater is $20^o$C when outside temperature is $-20^o$C and it is $10^o$C when the outside temperature is $-40^o$C. The temperature of the heater is

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. 80$^o$C

  2. 100$^o$C

  3. 40$^o$C

  4. 60$^o$C

Show answer
Correct Option: D
Explanation:

the temperature diffrence should be equal for equilibrium
$(t-20)=20-(-20)$;

so, $t-10=10-(-40)$
$t=60$

The absorptivity of Platinum black is

physics thermal properties heat transfer black body radiation kirchoff's laws

  1. 0.91

  2. 0.98

  3. 1.00

  4. 0.99

Show answer
Correct Option: B
Explanation:

The emissivity of Platinum black is 0.98. According to Kirchhoff's law of radiation, absorptivity is equal to emissivity at a given temperature. Hence, absorptivity = 0.98. 

Assertion : Good Reflectors are poor emitters of Thermal Radiation
Reason : The ratio of the emissive power and absorptive power is constant for all substances at any given temperature and radiation of same wavelength 

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. Assertion and Reason are correct and reason is the correct explanation for assertion

  2. Assertion and Reason are correct and reason is not the correct explanation for assertion

  3. Assertion is true but reason is false

  4. Assertion is false but reason is true

Show answer
Correct Option: A
Explanation:

Good reflectors are bad emitters of radiation.
Good reflectors, are called so because they absorb very little radiation and reflect the rest.
The ratio of absorptive and emissive power is constant for a given temperature and wavelength, so a bad absorber of radiation is also a bad emitter.
Hence the assertion and reason are true and the reason is the correct explanation.