Answers to Even-numbered Conceptual Questions
If an electric field exists in this region of space, and no magnetic field is present, the
electric field will exert a force on the electron and cause it to accelerate.
The magnetic field in the continental United States points primarily toward the north.
Therefore, an electron moving toward the east experiences a downward magnetic force.
Of course, the magnetic force on a positively-charged proton moving toward the east is
In this case, the magnetic force on the electron points to the east.
In each case, the force acting on the particle must point toward the center of curvature of
Therefore, particles 1 and 2 have negative charges; particle 3 has a positive
We want the magnetic force on the proton to be toward the center of the Earth, so that it
provides some of the necessary centripetal force.
It follows that the proton must move in
a westward direction.
In a uniform electric field, the force on a charged particle is always in the same direction,
leading to parabolic trajectories.
In a uniform magnetic field, the force of a charged
particle is always at right angles to the motion, resulting in circular or helical trajectories.
Perhaps even more important, a charged particle experiences a force due to an electric
field whether it is moving or at rest; in a magnetic field, the particle must be moving to
experience a force.
The electric field must point in the positive
direction, regardless of the sign of the
A current-carrying wire in a uniform magnetic field can experience zero force only if the
wire points in the same or opposite direction as the magnetic field.
In such a case, the
in Equation 22-4 will be either 0
, in which case
The force between wires carrying currents in the same direction is attractive, and inversely
proportional to the distance between the wires.
Similarly, the force between wires with
oppositely directed currents is repulsive.
It follows from simple geometry, then, that the
net force acting on wire 2 is directed toward wire 4.
If we apply the right-hand rule to wires 2 and 4, we see that they produce magnetic fields
at the center of the square that point toward wire 3.
On the other hand, the magnetic fields
produced by wires 1 and 3 at the center of the square cancel one another.
It follows that
the total magnetic field at the center of the square points toward wire 3.
If the current loop is to attract the magnet, it must produce a magnetic field with its north
pole pointing to the right; that is, pointing toward the south pole of the bar magnet.
this to be the case, the current in the wire must point out of the page, which means, in turn,
that terminal A must be the positive terminal.