Tag: laws of heat transfer

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Questions Related to laws of heat transfer

Multiple choice laws of heat transfer heat and thermodynamics physics

Which one of the following is the characteristic of Planck's quantum theory of radiation?

  1. The energy is not absorbed or emitted in integral multiple of quantum of energy.

  2. Radiation energy is not emitted or absorbed continuously but in the form of small packets of energy

  3. Radiation energy is emitted or absorbed continuously

  4. The magnitude of energy associated with a quantum is inversely proportional to its frequency
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

According to Planck's quantum theory, radiation is emitted in the form of small packets of energy and is not continuous. Quantum of energy can be expressed by the relation E=hv, where h is Planck's constant and v is the frequency.

Multiple choice laws of heat transfer heat and thermodynamics physics

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

  1. $0^oR$

  2. $0^oC$

  3. $0 K$

  4. $0^oF$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Prevost postulated that  radiative equilibrium is the condition where a steady state system is in dynamic equilibrium, with equal incoming and outgoing radiative flux and negligible heat transfer by conduction and convection.
So by extended logic, at $0K$ equilibrium cannot be reached.

Multiple choice laws of heat transfer heat and thermodynamics physics

Two spheres made of same material have radii in the ratio 2 : 1. If both the spheres are at same temperature, then what is the ratio of heat radiation energy emitted per second by them?

  1. 1 : 4

  2. 4 : 1

  3. 3 : 4

  4. 4 : 3
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Radiation emitted per second depends on the temperature of the body.
Stefan's law states that the rate of emission of radiant energy by unit area of perfectly black body is directly proportional to the fourth power of its absolute temperature.
      $E \propto AT^4$
or   $E \propto r^2$
($\because A= \pi r^2$ and T is same for both the spheres)
where r is radius of sphere.
$\frac{E _1}{E _2} = \frac{r^2 _1}{r^2 _2}$
$=\left(\frac{2}{1}\right)^2=\frac{4}{1}$
$=4:1$
Note : A black body at absolute temperature T surrounded by another black body at absolute temperature $T _0$ not only loses an amount of energy $\sigma T^4$, thus the amount of heat lost by the former per unit time is given by 
$E=\sigma (T^4-T _0^4)$
This law is stefan Boltzmann's law.

Multiple choice laws of heat transfer heat and thermodynamics physics

A temperature of a body is ${400^ \circ }$ C. Assuming the surrounding temperature to be negligible. At what temperature will body emit double energy radiation?

  1. ${200^ \circ }$ c

  2. 200 K

  3. ${800^ \circ }$ c

  4. 800 K
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Energy radiated E is proportional to T^4. If E' = 2E, then (T'/T)^4 = 2, so T' = T * 2^(1/4). T = 400 + 273 = 673 K. T' = 673 * 1.189 = 800 K. Converting back to Celsius is not required as the options provide Kelvin.

Multiple choice laws of heat transfer heat and thermodynamics physics

If the operating voltage of X-ray tube is $50$kV then velocity of X-ray?

  1. $7.5\times10^{25}$ m/sec

  2. $3\times 10^8$ m/sec

  3. $10^8$ m/sec

  4. $3$ m/sec
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

We know, wavelength of X-ray $\lambda =\dfrac { hv }{ eV } $

Where h is the Plank's constant and $v$ is the required velocity.
$v=\frac { \lambda \times eV }{ h } $
  the minimum wavelength of X-ray is 0.01 nm.
So velocity, $\ \quad \quad v=\frac { 0.01\times { 10 }^{ -9 }\times 50\times { 10 }^{ 3 } }{ 6.626\times { 10 }^{ -34 } } $
     v= $0.0754\times { 10 }^{ 28 }\quad m/s\ =7.5\times { 10 }^{ 25 }\quad m/s$


Multiple choice laws of heat transfer heat and thermodynamics physics

The hydrogen atom in its ground state is excited by means of monochromatic radiation of energy $12.75ev$. How many different lines are possible in the resulting spectrum? You may assume the ionization energy for hydrogen atom as $13.6\ eV$

  1. $3$

  2. $4$

  3. $6$

  4. $2$
Reveal answer Fill a bubble to check yourself
B Correct answer
Multiple choice laws of heat transfer heat and thermodynamics physics

If $c$ is the velocity of electromagnetic radiation $e$ is the charge of an electron $m$ is the mass of an electron and $h$ is the Planck's constant, then the combination of these universal constant that is dimensionless, is

  1. $me^{2}/(hc)$

  2. $hc/(me)$

  3. $mc^{2}/h$

  4. $None$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The fine structure constant alpha = e^2 / (4 * pi * epsilon_0 * h_bar * c) is dimensionless. The expression me^2 / (hc) is related to the Rydberg energy and is not dimensionless. None of the provided options are dimensionless.

Multiple choice laws of heat transfer heat and thermodynamics physics

The radiation corresponding to 3 $\rightarrow $ s 2 transition of hydrogen atom falls on a metal surface to produce photoelectrons. These electrons are made to enter a magnetic field of $3 \times 10 ^ { - 4 } T$ . If the radius of the largest circular path followed by these electrons is 100 mm, the work function of the metal is close to 

  1. 3.8 eV

  2. 1.1 eV

  3. 1.8 eV

  4. 1.6 eV
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$mv=qBR$

$K{E _{\max }} = \dfrac{{\left( {m{v^2}} \right)}}{{2m}} = 0.8\,eV$
$hv = 13.6\left[ {\dfrac{1}{4} - \dfrac{1}{6}} \right]$
$\therefore W = hv - K{E _{\max }}$
$ = 13.6 \times \dfrac{5}{{36}} - 0.8 = 1.1eV$
Hence,
option $(B)$ is correct answer.