RevIII-1 A charged particle is orbiting in a
uniform B-field in the sense shown. Is the
particle positive or negative?
A) positive
B) negative
C) impossible to tell
Two particles with the same charge but
different masses are moving in circular orbits in a magnetic field. They have the same
speed. Which one will have the larger radius orbit?
A) Neither, the orbits have the same size.
B) The larger mass particle will have a larger orbit.
C) The smaller mass particle will have a larger orbit.
Answers: The direction of acceleration and net force must be toward the center of the
circle. For CW rotation and B(in) and a negative particle, the force on particle is toward
the center of the circle.
The larger mass particle will have a larger orbit. You can see this from a physical
argument or from the formula for the radius of an orbit: R = mv/(qB). The physical
argument is this: both particles feel the same (sideways) size force F = qvB. The more
massive particle has more inertia and is therefore harder to turn, so it turns through a
larger orbit.
RevIII-2 A coil of wire carrying current I can rotate freely about an axis in a magnetic
field. If released from rest in the position shown, which way does it rotate?
A) right side will move out of page
B) left side will move out of page.
C) loop will not rotate at all
Answer: Right side will move out of
page. Use Fwire = I L B
B(in)
B
axis
I
