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:
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