CHAPTER 12 DRILL
- D I is false: The magnetic field lines due to a current-carrying wire encircle
the wire in closed loops. II is also false: Since the magnetic force is always
perpendicular to the charged particle’s velocity vector, it can do work on the
charged particle; therefore, it cannot change the particle’s kinetic energy. III,
however, is true: If the charged particle’s velocity is parallel (or
antiparallel) to the magnetic field lines, then the particle will feel no
magnetic force.
- C The magnitude of the magnetic force is FB = qvB, so the acceleration of the
particle has magnitude
- D By the right-hand rule, the direction of FB is out of the plane of the page
(since the particle carries a negative charge).
- E Since FB is always perpendicular to v, v cannot be upward or downward in
the plane of the page; this eliminates (B) and (C). The velocity vector also
cannot be to the right, as it is in (A), since then v would be antiparallel to B,
and FB would be zero. Using the right-hand rule, v must be into the plane of
the page.
- A The magnetic force provides the centripetal force on the charged particle.
Therefore,
- A The strength of the magnetic field at a distance r from a long, straight
wire carrying a current I is proportional to I /r; that is, B is inversely
