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Magnetic Field — Physics STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 SciencePhysicsMagnetic Field1 Mark
Question
If a charged particle at rest experiences no electromagnetic force:
  1. The electric field must be zero.
  2. The magnetic field must be zero.
  3. The electric field may or may not be zero.
  4. The magnetic field may or may not be zero.
✓

Answer

  1. The electric field must be zero.
  1. The magnetic field may or may not be zero.

Explanation:

Force on charged particle in an electric eld, $\text{F} = \text{qE} \ ...(1)$ 

Force on charged particle in a magnetic eld $\text{F} = \text{q} (\text{v}\times\text{b}) = \text{qvB} \sin\theta \ ...(2) $

Where boldface letter represent vector nature of that quantity, q is charge of the particle, v is the velocity of the particle( if any), and $\theta$ is the angle between velocity and magnetic eld. 

From (1), FE = 0 only when either q = 0 or E = 0. 

Let q ≠ 0, and F ≠ 0, then we must have E ≠ 0 

From (2), if q ≠ 0, v ≠ 0 and B ≠ 0 even then FB can be 'zero' because of θ = 0° or 180°

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