PS021
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Department of Physics
8.02
Fall 2008
Turn in at your table labeled with your name and group (e.g. L01 6B)
Problem Set 2
Due: Thursday, September 18 in 8.02 homework box
Warm Up
Problem 1: Charges on a Square
Three identical charges +
Q
are placed on the corners of a
square of side
a
, as shown in the figure.
(a) What is the electric field at the fourth corner (the one
missing a charge) due to the first three charges?
(b) What is the electric potential at that corner?
(c) How much work does it take to bring another charge
+
Q
from infinity to that corner?
(d) How much energy did it take to assemble the pictured configuration of three charges?
Problem 2:
Electric Potential
Suppose an electrostatic potential has a maximum at point P and a minimum at point M.
(a) Are either (or both) of these points equilibrium points for a negative charge?
If so are
they stable?
(b) Are either (or both) of these points equilibrium points for a positive charge?
If so are
they stable?
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Exam Problem
Problem 3: Charged Slab & Sheets
An infinite slab of charge carrying a charge
per unit volume
ρ
has a charged sheet
carrying charge per unit area
σ
1
to its left
and a charged sheet carrying charge per
unit area
σ
2
to its right (see top part of
sketch). The lower plot in the sketch shows
the electric potential
V
(
x
) in volts due to
this slab of charge and the two charged
sheets as a function of horizontal distance
x
from the center of the slab.
The slab is 4
meters wide in the
x
direction, and its
boundaries are located at
x =
2 meters and
x =
+2 meters, as indicated.
The slab is
infinite in the
y
direction and in the
z
direction (out of the page).
The charge sheets are located at
x =
6
meters and
x =
+6 meters, as indicated.
(a) The potential
V
(
x
)
is a linear function
of
x
in the region
6 meters <
x <
2
meters.
What is the electric field in this
region?
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 Fall '08
 SCIOLLA

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