Class 10 > Unit # 17:Introductory Electronics > Deflection of electron in magnetic field

Talha's Physics Academy

Deflection of Electron in a Magnetic Field

Deflection of an Electron Beam in a Magnetic Field

"A beam of moving electrons can be deflected by a magnetic field generated by passing a current through a pair of coils or magnets."

In diagrams, a uniform magnetic field directed perpendicularly into the page is represented by crosses ($\times$). When a moving electron enters this magnetic field, it experiences a magnetic Lorentz force that acts at right angles ($90^\circ$) to the direction of its motion. If the direction of the magnetic field is reversed, the direction of the resulting force is also reversed. The direction of this force is determined using Fleming's Left-Hand Rule.

Key Effects of Magnetic Field Deflection

1. Direction Change: The electron beam bends and changes its direction of travel.
2. Circular Trajectory: Because the magnetic force acts continuously perpendicular to the velocity vector (serving as a centripetal force), the electron moves in a circular path inside the magnetic field.
3. Constant Energy: The magnetic force does no work on the electrons because it is always perpendicular to their displacement; consequently, the energy of the electron beam does not change.
4. Constant Speed: Since kinetic energy remains constant, the speed of the electron beam does not change while traveling through a static magnetic field.

Note on Distinction: Unlike an electric field (which changes electron speed and follows a parabolic path), a magnetic field bends electrons into a circular path at constant speed.

Fig: Deflection of an electron beam following a circular path in a uniform magnetic field directed into the page ($\times$).

© 2026 Talha's Physics Academy. All rights reserved.