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

Since the magnetic field is into the page and the charge is moving clockwise, it means that the magnetic force must be directed towards the center of the spiral. Since the radius of the spiral is decreasing, this means that the magnitude of the magnetic force is decreasing. Therefore, the charge must be slowing down. Furthermore, since the force is directed towards the center of the spiral, the charge must be negative (as the force on a positive charge would be directed outwards). Thus, the best choice is B.

A Uniform Magnetic Field Is Directed into the Page