Exam #2 (Electricity)
Physics 1220 – Spring 1220
04/15/2010
Prof. Adrian Feiguin
(your name)
Problems: (1 pt. each. You can choose 6 out of 7, or lowest score will be dropped if you
complete all of them).
USE SI UNITS TO EXPRESS YOUR RESULTS
1)
Three point charges q1,q2,q3, with q2=q1, and q1=2x10
7
C, are arranged equally spaced along
a horizontal line, as shown in the figure. Calculate the sign and magnitude of q3, such that the
resulting force on q1 is zero.
2)
Two positive point charges of magnitude q1=2x10
7
C and q2=3x10
7
C are separated by a
distance of 0.1m. Calculate the electric field and the electric potential at:
a)
The midpoint between the charges.
b)
A point located at 0.1 m from both charges.
3)
Consider a close surface shaped as a cube of size
a
, as shown in the figure. There is an electric
field going through the cube along the ydirection that varies linearly as
E=C.y
, where
C
is
some constant. Find the flux through the surface of the cube, and the total charge inside as a
function of
a
and
C.
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 Fall '10
 Feiguin
 Electric Potential, Electrostatics, Potential difference, Electric charge, Surface charge distribution

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