Question:medium

A charged particle travels along a straight line with a speed $\nu$ in a region where both electric field $\mathbf{E}$ and magnetic fields $\mathbf{B}$ are present. It follows that}

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In a velocity selector, $\mathbf{E}$, $\mathbf{B}$, and $\mathbf{v}$ are mutually perpendicular with $v = E/B$.
Updated On: May 3, 2026
  • $\mathbf{E} = \nu \mid \mathbf{B}$ and the two fields are parallel
  • $\mathbf{E} = \nu \mid \mathbf{B}$ and the two fields are perpendicular
  • $\mathbf{B} = \nu \mid \mathbf{E}$ and the two fields are parallel
  • $\mathbf{B} = \nu \mid \mathbf{E}$ and the two fields are perpendicular
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The Correct Option is B

Solution and Explanation

A charged particle moving in a straight line within a region containing both electric field \(\mathbf{E}\) and magnetic field \(\mathbf{B}\) presents us with an interesting scenario. We know that the force acting on a charged particle in the presence of electric and magnetic fields is given by the Lorentz force equation:

\(\mathbf{F} = q(\mathbf{E} + \mathbf{v} \times \mathbf{B})\)

Where:

  • \(\mathbf{F}\) is the total force on the particle.
  • \(q\) is the charge of the particle.
  • \(\mathbf{v}\) is the velocity of the particle.
  • \(\mathbf{E}\) is the electric field.
  • \(\mathbf{B}\) is the magnetic field.

For the particle to travel in a straight line, the net force \(\mathbf{F}\) acting on it must be zero. Thus, we have:

\(q(\mathbf{E} + \mathbf{v} \times \mathbf{B}) = 0\)

Since \(q \neq 0\) (as it is a charged particle), we simplify to:

\(\mathbf{E} + \mathbf{v} \times \mathbf{B} = 0\)

Rearranging gives:

\(\mathbf{E} = -(\mathbf{v} \times \mathbf{B})\)

This indicates that the electric field \(\mathbf{E}\) is equal and opposite to the vector cross product of the velocity \(\mathbf{v}\) and magnetic field \(\mathbf{B}\).

For the cross product \(\mathbf{v} \times \mathbf{B}\) to be non-zero, \(\mathbf{v}\) and \(\mathbf{B}\) must be perpendicular to each other.

Hence, the correct option that satisfies these conditions is:

\(\mathbf{E} = \nu \mid \mathbf{B}\) and the two fields are perpendicular

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