proportional to r. So, at the distance from the wire, the magnetic field will
be twice as strong.
- E The magnetic force provides the centripetal force on the charged particle. It
follows that
Increasing m or v, or decreasing q or B, would make r larger, not
smaller. Therefore, the answer must be (E).
- D The magnetic field B cannot be in the plane of the page (because both v and
FB are), so we can eliminate (A), (B), and (C). If B pointed into the plane of
the page, it’s easy to see that the right-hand rule tells us that FB would point
upward in the plane of the page. Therefore, B must point out of the plane of
the page.
- E Since v is perpendicular to B, the strength of the magnetic force, FB, is just
qvB, where q is the magnitude of the charge. In this case, then, we find that
FB = (0.04 C)(2 × 10^4 m/s)(0.5 T) = 400 N
