Tag: thermal properties

Quizzes (8) Questions (112)

Questions Related to thermal properties

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

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

  1. $E/A$

  2. $EAT$

  3. $EA/T$

  4. $A/E$
Reveal answer Fill a bubble to check yourself
A Correct answer
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)  = $\dfrac{E}{A}$

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

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

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

  2. $p - q$

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

  4. $q + p$
Reveal answer Fill a bubble to check yourself
C Correct answer
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=$\cfrac{q}{p}$
Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

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

  1. $(q-p)/p$

  2. $q/p$

  3. $(p-q)/p$

  4. $p/q$
Reveal answer Fill a bubble to check yourself
C Correct answer
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 = $\dfrac{p-q}{p}$

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

Kirchoffs law states that

  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
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

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

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

  1. $X+Y$

  2. $XY$

  3. $Y/X$

  4. $X/Y$
Reveal answer Fill a bubble to check yourself
C Correct answer
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 = $\dfrac{Y}{X}$

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

Good absorbers of heat are 

  1. Poor emitters of heat

  2. Good emitters of heat

  3. Only black bodies

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
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.
Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

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

  1. 80$^o$C

  2. 100$^o$C

  3. 40$^o$C

  4. 60$^o$C
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

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

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

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

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 

  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
Reveal answer Fill a bubble to check yourself
A Correct answer
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.