Tag: electromagnetic forces

Questions Related to electromagnetic forces

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.

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

  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

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Correct Option: A

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 $(h=6.6\times { 10 }^{ -34 }Js)$

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

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

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

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

  4. $4\times { 10 }^{ 18 }$

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Correct Option: B
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

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 :

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

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

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

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

  4. $11.2\times 10^{4}\ A$

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Correct Option: C
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 } $

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.

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

  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

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Correct Option: D

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 :

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

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

  2. $0$

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

  4. $(\pi\times R/2) \times (Bev)$

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Correct Option: B
Explanation:

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

Velocity ratio of the two cathode rays is 1:2. They are applied to the same electric field. What is the deflection ratio of the two cathode rays

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

  1. 1 : 2

  2. 1 : 4

  3. 4 : 1

  4. 8: 1

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Correct Option: A

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  $=9\times 10^{ { -31 } }{ kg }$ )

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

  1. $1 cm$

  2. $5 cm$

  3. $10 cm$

  4. $25 cm$

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Correct Option: B

The minimum amount of energy released in annihilation of electron-positron is.

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

  1. $1.02\ MeV$

  2. $0.58\ MeV$

  3. $185\ MeV$

  4. $200\ MeV$

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Correct Option: A
Explanation:
Energy released during the annihilation of ${ e }^{ - },{ e }^{ + }$ pair is,
$E=2{ m } _{ c }{ C }^{ 2 }$
$E=2\times 9.1\times { 10 }^{ -31 }\times { \left( 3\times { 10 }^{ 8 } \right)  }^{ 2 }J$
$E=\dfrac { 2\times 9.1\times 9\times { 10 }^{ -15 } }{ 1.6\times { 10 }^{ -13 } } MeV$
$E=1.02MeV$

A spherical drop of radius $10$ has absorbed $40$ electrons. The energy required to given an additional electron to it is.

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

  1. $9.2 \times 10^{-21} J$

  2. $5.7 \times 10^{-21} J$

  3. $9.21 \times 10^{-23} J$

  4. $4Ke^2$

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Correct Option: D
Explanation:
We know that,
Energy $=$ Potential $\times $ Charge
$E=\left( \dfrac { k\quad q }{ r }  \right) \times e$
$E=\dfrac { k\left( 40e \right) \left( e \right)  }{ 10 } $
$E=4k{ \left( e \right)  }^{ 2 }$
$\boxed { Energy=4{ ke }^{ 2 } } $

Cathode rays enter an electric field normal to the field. Then their path in the electric field is :

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

  1. A parabola

  2. A circle

  3. A straight line

  4. An ellipse

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Correct Option: A
Explanation:

Electric field applies a constant force  $eE$ on the electrons (i.e. cathode rays) perpendicular to the beam of electrons due to which the electrons will trace a parabolic path.