Physics 132-01—Physics IIa: Electricity and Magnetism
Homework Assignment #7
Due Tuesday, March 31, 2009, 1:10 p.m.
NOTE SPECIAL DUE DATE AND TIME!
Do the On-line Assignment “HW #7.”
Pencil-and-Paper problems:
Do the following problems:
Chapter 29: Problem 58. Also, for this problem, plot the magnetic field as a func-
tion of xfor xfrom −sto +s. As in previous problems calling for a plot, you may use
Excel,Mathematica, any similar tool, or even a hand-drawn sketch, as long as you give a
reasonable curve and indicate the scales appropriately on the axes.
Also, do the following supplemental problem.
S1. (30 pts.) A manufacturer of electric blankets claims that a new wiring pattern reduces
the electromagnetic field by 95%. Is this plausible? Explain your reasoning. If you make
any assumptions or approximations, be explicit about them. You might wish to consider
the following points:
1. The current in an electric blanket is AC (alternating current), but you will come to
the same conclusion if you proceed as if it were DC (direct current).
2. Since the blanket is electrically neutral, the electric field is very small, and you may
restrict your attention to the magnetic field.
3. The original claim is made in terms of a percent change, so you will probably find it
easiest to deal with ratios.
4. Assume that the heating elements are made of resistive wire coiled into circles with
a radius of about 10cm. Compare the magnetic fields at the center of the following
two configurations. The spacing between the two circles in the second configuration
is limited only by the thickness of the wires. You may assume that the wire is of
about the same thickness as ordinary wire going to something like an electric lamp.
Please write NEATLY and show your work CLEARLY. I won’t give you credit if I
can’t follow your work. Staple your pages together.
