Question

A charged particle travels along a straight line with a speed $v$ in a region where both electric field $E$ and magnetic field $B$ are present. It follows that

• A. | E | = v | B | and the two fields are parallel
• B. | E | = v | B | and the two fields are perpendicular
• C. | B | = v | E | and the two fields are parallel
• D. | B | = v | E | and the two fields are perpendicular

Answer 1

Answer: (B)

Force due to magnetic field is
$F=qvB\sin \theta$
When,
$\theta =90^{\circ }$
$F=qvB\dots$(i)
Force due to electric field $E$ is
$F=qE\dots$(ii)
Equating Eqs. (i) and (ii), we get
$|E|=v|B|$