CBSE Class 12-science Answered

Let us consider an ion beam having charge +q and velocity  moving along x-direction under the combined influence of uniform electric field  acting along y-axis and a uniform magnetic field  along z-axis.

In other way we have,

Due to electric field the ion beam experiences a force,

Due to magnetic field the ion beam experiences a force,              

Thus, electric and magnetic forces are in opposite directions.

 If the values of  and  are so adjusted that the magnitudes of the two forces are equal, the two equal and opposites forces will nullify each other. The result is that the net force acting on ion beam will become zero and the beam will travel along a straight line without suffering any deviation.

This happens when   q E = B qv

This condition can be used to select charged particle of a particular velocity out of a beam containing charges moving with different speeds. Because for a given pair of electric and magnetic field ions with a particular velocity will go undeviated and all other ions are deflected. Such an arrangement is called velocity selector.