Question

A uniform magnetic field is directed into the page. A charged particle moving in the plane of the page follows a clockwise spiral of decreasing radius as shown in the given figure. Which of the following statements is correct ?

• 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 B The charge is negative and slowing down

The charge is negative because $qv\times B$ determines the direction of acceleration.
$F=qvB=\frac{m{v}^2}{r}.Thatis,v=\frac{qB}{m}R$.
That is, $v$ is proportional to $R$ (the instantaneous radius of the spiral), since $R$ is decreasing, so is $v$.
Hence, the charge is slowing down in speed.