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Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

What is the work function of tungsten at $1500\ K$ temperature, when a diode valve with a tungsten filament works at $1500\ K$? Assume the work function of tungsten at $0\ K$ is $4.52\ eV$:

  1. $4.71\ eV$

  2. $0.39\ eV$

  3. $8.86\ eV$

  4. $1.25\ eV$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The work function at temperature T is given by phi(T) = phi(0) + 2kT. Given phi(0) = 4.52 eV, k = 8.617 * 10^-5 eV/K, and T = 1500 K, the increase is 2 * 8.617 * 10^-5 * 1500 = 0.2585 eV. 4.52 + 0.26 = 4.78 eV. Option A is the closest.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Metallic sphere of radius is removed from other bodies,  and is irradiated with the radiation of wavelength Lambda, so that it is capable of causing Photoelectric effect.

  1. The maximum potential gained by the sphere will be independent of its radius

  2. Net positive charge appearing on the sphere after a long time will depend on the radius of the sphere

  3. The kinetic energy of the most energetic electron emanating from the Sphere will keep on declining with time

  4. The kinetic energy of the most energetic electrons emitted from this sphere initially will be independent of the radius of the sphere
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Monochromatic light of wavelength 440 nm is produced. The power emitted by light is 18 mW. The number of photons emitted per second by light beam is $(h=6.6\times { 10 }^{ -34 }Js)$

  1. $3\times { 10 }^{ 16 }$

  2. $4\times { 10 }^{ 16 }$

  3. $3\times { 10 }^{ 18 }$

  4. $4\times { 10 }^{ 18 }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given,

$\lambda=440nm$
$h=6.6\times 10^{-34} Js$
$I=18m W$
The energy of monochromatic light,
$E=\dfrac{hc}{\lambda}$
$E=\dfrac{6.6\times 10^{-34}\times 3\times 10^8}{440\times 10^{-9}}$
$E=0.045\times 10^{-17}J$
The number of photon emitted per second by the light beam,
$n=\dfrac{I}{E}$ 
$n=\dfrac{18\times 10^{-3}}{0.045\times 10^{-17}}$
$n=4\times 10^{16}$
The correct option is B

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

In a hydrogen atom, an electron moves in an orbit of radius $5.0\times 10^{-11}\ m$ with a speed of $2.2\times 10^{6}\ ms^{-1}$. The equivalent current is :

  1. $11.2\times 10^{-3}\ A$

  2. $1.9\times 10^{-3}\ A$

  3. $1.12\times 10^{-3}\ A$

  4. $11.2\times 10^{4}\ A$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Current $=$ Charge passing per unit time $q$
$I=\dfrac { q }{ t } $
$t=\dfrac { 2\pi r }{ v } $
$I=\dfrac { q }{ 2\pi r } .v=\dfrac { 1.6\times { 10 }^{ 19 }\times 2.2\times { 10 }^{ 6 } }{ 5\times { 10 }^{ -11 }\times 2n } $
$I=\dfrac { 1.6\times 2.2 }{ 5\times 2\pi  } \times { 10 }^{ -2 }=1.12\times { 10 }^{ -3 }A$
$\boxed { I=1.12mA } $
Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Statement -1 :When ultraviolet light is incident on a photo cell, its potential is $ V _0 $ and the maximum kinetic energy of the photoelectrons is $ K _{max} $. When the ultravoilet light is replaced by X-rays, both $ V _0 $ and $ K _{max} $ increase. 

Statement-2: Photoelctrons are emitted with speeds ranging from zero to a maximum value because of the range of frequencies present in the incident light.

  1. Statement -1 is true,statement-2 is true ; statement -2 is correct explanation of statement-1.

  2. Statement -1 is true,statement-2 is true ; statement -2 is not correct explanation of statement-1.

  3. Statement -1 is false,statement-2 is true

  4. Statement -1 is true,statement-2 is false
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Statement 1 is true because X-rays have higher frequency than UV light, increasing the photon energy and thus Kmax and stopping potential. Statement 2 is false because the range of speeds is due to electrons being emitted from different depths within the material, not the frequency range of the light.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

If an electron covers 1/4th of revolution in a circle having radius R and in the presence of perpendicular magnetic field B, the amount of energy acquired by electron will be :

  1. $\dfrac{1}{4}mv^2$

  2. $0$

  3. $\dfrac{1}{8}mv^2$

  4. $(\pi\times R/2) \times (Bev)$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Magnetic Force is perpendicular to electron's velocity, so it can only change direction of electron's velocity but cannot change its acceleration

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

In a photocell bichromatic light of wavelengths  $2475 \mathring { A } $  and  $6000 \mathring { A } $  are incident on cathode whose work function is  $4.8 e V.$  If a uniform magnetic field of  $3 \times 10 ^ { - 5 }$  Tesla exists parallel to the plate, the radius of the path describe by the photoelectron will be (mass of electron  $=9\times 10^{ { -31 } }{ kg }$ )

  1. $1 cm$

  2. $5 cm$

  3. $10 cm$

  4. $25 cm$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The radius of the path of a charged particle in a magnetic field is given by r = mv/qB. Using the photoelectric equation, the kinetic energy is E = hc/lambda - work function. Calculating for the shorter wavelength (2475 A) gives a kinetic energy of 0.2 eV, which leads to a radius of 5 cm.