Question

A charged particle is whirled in a horizontal circle on a frictionless table by attaching it to a string fixed at one end. If a magnetic field is switched on in the vertical direction, the tension in the string

• A. will increase.
• B. will decrease.
• C. remains same.
• D. may increase or decrease.

Answer 1

Answer: (D)

Let the string makes an angle $\theta$ with the vertical.

Initially, before the magnetic field is applied, $T\sin \theta =\frac{m{v}^2}{r}$

If the particle experiences a force outward in the direction of the radius, tension $T$ will increase.
If the particle experiences a force inward in the direction of the radius towards the center, tension $T$ will decrease.