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  • The monoenergetic beam of electrons moving along +y direction enters a region of uniform electric and magnetic fields. If the beam goes straight undeflected, then fields B and E are

The monoenergetic beam of electrons moving along +y direction enters a region of uniform electric and magnetic fields. If the beam goes straight undeflected, then fields B and E are directed respectively along

Option: 1

-y axis and -z axis


Option: 2

+z axis and +x axis


Option: 3

+x axis and +z axis
 


Option: 4

-x axis and -y axis


Answers (1)

best_answer

The total Lorentz force on the electron is
\mathbf{F}=-e(\mathbf{E}+\mathbf{v} \times \mathbf{B})
The electron will be undeflected if \mathbf{v} \perp \mathbf{B}~ If ~\mathbf{E}  is along +z-direction, the force -e \mathbf{E} will be along -z-direction. If \mathbf{B} is along +x direction, force \mathbf{v} \mathbf{\times \mathbf{B}}  will be along +z direction. When \mathrm{e E=e v B} , the electron moves along +y-direction undeflected. Hence the correct choice is (c). Thus, for an electron moving along +y direction, the electric field should be along +z direction and magnetic field along +x direction, then the electron will be undeflected.

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HARSH KANKARIA

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