Tag: physics

Questions Related to 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?

laws of heat transfer heat and thermodynamics physics

  1. 1 : 4

  2. 4 : 1

  3. 3 : 4

  4. 4 : 3

Show answer
Correct Option: B
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.

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?

laws of heat transfer heat and thermodynamics physics

  1. ${200^ \circ }$ c

  2. 200 K

  3. ${800^ \circ }$ c

  4. 800 K

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Correct Option: D

If the temperature of a hot body is increased by $50\%$, then the increase in the quality of emitted heat radiation will be

laws of heat transfer heat and thermodynamics physics

  1. $125\%$

  2. $200\%$

  3. $300\%$

  4. $400\%$

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Correct Option: B

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

laws of heat transfer heat and thermodynamics physics

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

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

  3. $10^8$ m/sec

  4. $3$ m/sec

Show answer
Correct Option: A
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= $0.0754\times { 10 }^{ 28 }\quad m/s\ =7.5\times { 10 }^{ 25 }\quad m/s$


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 $13.6\ eV$

laws of heat transfer heat and thermodynamics physics

  1. $3$

  2. $4$

  3. $6$

  4. $2$

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Correct Option: B

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

laws of heat transfer heat and thermodynamics physics

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

  2. $hc/(me)$

  3. $mc^{2}/h$

  4. $None$

Show answer
Correct Option: D

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 

laws of heat transfer heat and thermodynamics physics

  1. 3.8 eV

  2. 1.1 eV

  3. 1.8 eV

  4. 1.6 eV

Show answer
Correct Option: B
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.

Two black metallic spheres of radius 4m, at 2000 K and 1m at 4000 K will have ratio of energy radiation as

laws of heat transfer heat and thermodynamics physics

  1. 1 : 1

  2. 4 : 1

  3. 1 : 4

  4. 2 : 1

Show answer
Correct Option: A

Two sphere of same material and of same emissivity have radii 1 m and 4 m and temperature 4000 K and 1000 K, respectively. The ratio of radiation emitted per sec is  

laws of heat transfer heat and thermodynamics physics

  1. 4:1

  2. 1:4

  3. 1:1

  4. 16:1

Show answer
Correct Option: C

We find that the temperature of air decreases as one goes up from the earth's surface because

laws of heat transfer heat and thermodynamics physics

  1. The atmospheric pressure drops with height

  2. The earth which radiates in the infrared region is the main heat source and temperature drops as we go away from it

  3. The density of air drops with height and the air therefore cannot hold stronger as we go up

  4. Winds are stronger as we go up

Show answer
Correct Option: B
Explanation:
The behaviour of atmosphere is different for visible and infrared radiations. Most of the infrared radiations are not allowed to pass through atmosphere, since it reflects them. The energy from the sun, heats the earth which in turn starts emitting radiations. Since, the earth gets heated to much lower temperature than the temperature of the sun, according to Planck's law, the radiations emitted by the earth are mostly in the infrared region. These radiations from the earth are not allowed to pass through the atmosphere which reflects them back. As a result of it the earth's atmosphere becomes richer in infrared radiations which are sometimes called heat radiations. So, earth radiates in the infrared region, this is the source of heat. As we go up, it decreases.