Question

The path of cathode rays in an electric field can be approximated to a circle of radius r. In order to double the radius of the circular path, we must

• A. reduce the electric field to half
• B. double the electric field
• C. increase electric field four times
• D. reduce electric field to one fourth

Answer 1

The correct option is B double the electric field

Given, path of cathode rays is nearly circular. So, electric force must be acting as centripetal force for the cathode rays and it is given by
$F_e=F_c$
qE = $m{\omega }^{2}r$
we can conclude that $E\alpha r$ ,provided all the other quantities are constant.
Thus, to double the radius we should double electric field strength.