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JEE Main 2025 Apr 2 Shift 1, Physics Q20: Introduction and Photoelectric Effect

JEE Main2025Apr 2, Shift 1Physics
Q.

A monochromatic light is incident on a metallic plate having work function . An electron, emitted normally to the plate from a point A with maximum kinetic energy, enters a constant magnetic field, perpendicular to the initial velocity of electron. The electron passes through a curve and hits back the plate at a point B. The distance between A and B is :

(Given : The magnitude of charge of an electron is and mass is , is Planck's constant and is velocity of light. Take the magnetic field exists throughout the path of electron)

  1. A

  2. B

  3. C

  4. D

Solution

Maximum kinetic energy via Einstein's photoelectric equation:

Momentum of the ejected electron:

Circular motion in the magnetic field. The electron leaves the plate perpendicular to the surface, then is bent by the perpendicular magnetic field into a circle of radius:

Since the velocity is perpendicular to the plate at the start, the electron travels a semicircle and lands back on the plate. The chord AB equals the diameter of the circle:

Option (3) is correct.

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