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. A reasonable explanation is:
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
E) none of the above

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. A reasonable explanation is: 
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 
E) none of the above

Quizplus · Accepted Answer

The spiral motion is caused by the magnetic force, which is always perpendicular to the velocity of the charged particle. As the particle spirals inward, its speed must be decreasing, and since the magnetic force is always perpendicular to the velocity, the acceleration due to the magnetic force must also be perpendicular to the velocity. This means that the magnetic force must be acting towards the center of the spiral, which requires the charge to be negative. Therefore, the best choice is B, the charge is negative and slowing down.

A Uniform Magnetic Field Is Directed into the Page