©1997 by Eric Mazur
Published by Pearson Prentice Hall
Upper Saddle River, NJ 07458
ISBN 0-13-565441-6
No portion of the file may be distributed, transmitted in any form, or included in other documents
without express written permission from the publisher.
Electrodynamics
As the capacitor shown below is charged with a constant
current I, at point P there is a
1. constant electric field.
2. changing electric field.
3. constant magnetic field.
4. changing magnetic field.
5. changing electric field and a magnetic field.
6. changing magnetic field and an electric field.
7. none of the above.
For a charging capacitor, the total displacement current
between the plates is equal to the total conduction current I
in the wires. The capacitors in the diagram have circular
plates of radius R. In (a), points A and B are each a distance
d > R away from the line through the centers of the plates; in
this case the magnetic field at A due to the conduction
current is the same as that at B due to the displacement
current. In (b), points P and Q are each a distance r < R away
from the center line. Compared with the magnetic field at P,
that at Q is
1. bigger.
2. smaller.
3. the same.
4. need more information.
A planar electromagnetic wave is propagating through space.
Its electric field vector is given by E = Eo cos(kz – wt) Its
magnetic field vector is
At a fixed point, P, the electric and magnetic field vectors in
an electromagnetic wave oscillate at angular frequency w. At
what angular frequency does the Poynting vector oscillate
at that point?
1. 2w
2. w
3. w /2
4. 4w
Which gives the largest average energy density at the
distance specified and thus, at least qualitatively, the best
illumination
1. a 50-W source at a distance R.
2. a 100-W source at a distance 2R.
3. a 200-W source at a distance 4R.