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 charged particle is moving in a spiral of decreasing radius, which means that its speed is decreasing. In a uniform magnetic field, the force on a charged particle is perpendicular to both its velocity and the magnetic field. Since the particle is moving clockwise and the magnetic field is directed into the page, the force on the particle is directed outward (i.e. away from the center of the spiral). This means that the charge must be negative, since the force on a positive charge would be directed inward. Furthermore, since the particle's speed is decreasing, its kinetic energy is decreasing, and it must be losing energy. Therefore, the particle is slowing down.

A Uniform Magnetic Field Is Directed into the Page