Question

4. The next three questions pertain to a region where there is a uniform magnetic field directed toward north.

On which of the following particles will the force exerted by the magnetic field be not zero?

a a proton moving toward south.

b an electron moving toward north.

c an electron moving toward east.

d a neutron moving toward south.

e a neutron moving toward west.

Answer 1

Correct option is : (c) An electron moving toward east.

Reason : Force exerted on a particle having charge q and velocity v, moving in a magnetic field of intensity B is F = qvB sin $\theta$ , where, $\theta$ is the angle between velocity vector
ין
and the magnetic field vector
B
.

So, to have a non - zero force, q must be non - zero, and, $\theta$ must be not equal to 0^o or 180^o, so that, sin $\theta$ remains non - zero.

For a proton or electron moving toward north, $\theta$ = 0^o, and, that moving toward south, $\theta$ = 180^o. So, F = 0.

Also, for a neutron, q = 0. So, F = 0.

Hence, the only option for having a non - zero magnetic force is an electron moving toward east, since, q is non - zero for an electron, and, $\theta$ = angle between east and north = 90^o in this case, so, sin $\theta$ = sin 90^o = 1.