Moving Charges and Magnetism - Result Question 23

23. A beam of electrons is moving with constant velocity in a region having simultaneous perpendicular electric and magnetic fields of strength $20 Vm^{-1}$ and $0.5 T$ respectively at right angles to the direction of motion of the electrons. Then the velocity of electrons must be

[1996]

(a) $8 m / s$

(b) $20 m / s$

(c) $40 m / s$

(d) $\frac{1}{40} m / s$

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Answer:

Correct Answer: 23. (c)

Solution:

  1. (c) The electron moves with constant velocity without deflection. Hence, force due to magnetic field is equal and opposite to force due to electric field.

$q v B=q E \Rightarrow v=\frac{E}{B}=\frac{20}{0.5}=40 m / s$