Question

A uniform magnetic field is directed out of the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius as shown, then

• A. the charge is positive and slowing down
• B. the charge is negative and slowing down
• C. the charge is positive and speeding up
• D. the charge is negative and speeding up

Answer 1

The correct option is A the charge is positive and slowing down

$R=\frac{mv}{qB};\overrightarrow{F}=q(\overrightarrow{v}\times \overrightarrow{B})$

As $R$ decreases, $v$ will decrease.

Clockwise motion for upward magnetic field $\to$ +ve charge.