Question

The path of the cathode rays in an magnetic field can be approximated to a circle. In order to double the radius of the circular path keeping velocity constant :

• A. double the magnetic field
• B. halve the magnetic field
• C. increase the magnetic field to four times
• D. triple the magnetic field

Answer 1

The correct option is B halve the magnetic field

The cathode rays follow a circular path due to the centripetal force of the magnetig field acting on the rays.

$\therefore \frac{m{v}^2}{r}=qvB$

Dividing both sides by 2, we get

$\frac{m{v}^2}{2r}=\frac{qvB}{2}=qv\times \frac{B}{2}$

Hence the field should be halved