CHAPTER 13 DRILL
- E Since v is upward and B is out of the page, the direction of FB on the free
electrons will be to the left, leaving an excess of positive charge at the right.
Therefore, the potential at point b will be higher than at point a, by ε = vBL
(motional emf).
- C If the plane of a loop enclosing an area A is perpendicular to a uniform
magnetic field of strength B, then the magnetic flux through the loop, ΦB, is
simply equal to the product BA. Since both these loops have the same area,
the magnetic fluxes through them will be the same, regardless of their shape.
- C To the right of the long straight wire, the magnetic field points into the plane
of the page, and it is stronger at x 1 than at x 2. This means that the “into-the-
page magnetic flux” through the loop increases as the loop is moved from x 1
to x 2. Since the flux is into the page and increasing, Lenz’s law says that the
current induced in the loop will be counterclockwise to produce its own out-
of-the-page magnetic flux.
- E A change in the magnitude of a magnetic field requires either a change in the
magnitude of the current, or a change in the area of a current coil. Since the
current in the straight wire is steady, there is no change in the magnetic field,
no change in magnetic flux, and, therefore, no induced emf or current.
- C By definition, magnetic field lines emerge from the north pole and enter at
the south pole. Therefore, as the north pole is moved upward through the
loop, the upward magnetic flux increases. To oppose an increasing upward
flux, the direction of the induced current will be clockwise (as seen from
above) to generate some downward magnetic flux. Now, as the south pole
moves away from the center of the loop, there is a decreasing upward
magnetic flux, so the direction of the induced current will be
counterclockwise.
