Question

In a region of space cathode rays move along +ve Z-axis and a uniform magnetic field is applied along X-axis. If cathode rays pass undeviated, the direction of electric field will be along

• A. -ve X-axis
• B. +ve Y-axis
• C. -ve Y-axis
• D. +ve Z-axis

Answer 1

Answer: (C)

-ve Y-axis

If the charged particle enters normally into the magnetic field, the force $F=Bqv$ and its direction is perpendicular to both the directions of the velocity of the charged particle and the magnetic field as given by Fleming's left hand rule.

Hence the charged particle follows a circular path of radius r for which the necessary centripetal force $\frac{m{v}^2}{r}$ is given by the magnetic force $Bqv.$

Hence radius of the circular path $r=\frac{mv}{qB}$.

From this, we can say that the resultant magnetic force acting in +ve Y axis. For the rays to not deflect the electric field should be directed along -ve Y axis.