Q: 11.21 (b) Is the formula you employ in (a) valid for calculating radius of the path of a electron beam? If not, in what way is it modified?
The formula used in (a) can not be used. As the electron would be travelling at a very high speed we can not take its mass to be equal to its rest mass as its motion won't be within the non-relativistic limits.
The value for the mass of the electron would get modified to
where m is the relativistic mass, m0 is the rest mass of the body, v is the very high speed at which the body is travelling and c is the speed of light.
The radius of the circular path would be