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PHYSICS 208
EXAM A
SEPTEMBER 21, 2010
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(10) 1. An electric field is set up between two parallel plates, each of area 2.0 m
, by putting 1.0 x 106 C charge on one
2
plate and a 1 x 106 C charge on the other. The plates are separated by 4.0 mm. What is the magnitude of the electric
field between the plates at a distance of 1.0 mm from the positive plate?
N/C; c. 4.2 x
N/C; d. 5.6 x
a. 0 N/C; b. 1.4 x
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(10) 2. If the force between two charges has magnitude 3.36 N when the charges are separated by 8.0 mm, the
magnitude of the force between these two charges when separated by 12 mm is
a. 7.56 N; b. 0.023 N; c. 1.68 N; d. 1.49 N;
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(10) 3. If a rectangular area is rotated in a uniform electric field from the position where the maximum electric flux goes
through it to an orientation where only half the flux goes through it, what has been the angle of rotation?
a. 30 degrees; b. 45 degrees; c. 60 degrees; d. 26.6 degrees;
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(10) 4. A uniform spherical shell of charge of radius R surrounds a point charge at its center. The point charge has a
value Q and the shell has total charge minus Q. The electric field at a distance R/2 from the center
a. is zero; b. does not depend on the charge of the spherical shell; c. is half of what it would be if only the point charge
were present; d. is directed inward if Q> 0;
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(10) 5. Four point charges, each of charge 2.5 x i0 C, are located on the x and y axes, one at each of the locations
(0,2m), (0, -2m), (2m, 0), and (-2m, 0). The potential at the origin is
a. 0; b. 1.13 x
V; c. 2.25 x i0
5 V;
5 V; d. 4.50 x i0
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(10) 6. The potential halfway between two equal charges Q separated by a distance d is
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(20) 7. An isolated, large conducting plate has a charge per unit area a
=
106 C/rn
2 on its surface (see the figure).
a. Use Gauss’s Law to find E at a point 1 cm from the plate. (Draw the Gaussian Surface and start with the Gauss’s
Equation showing each step clearly.)
b. A proton (charge +1.6x 1019 C and mass 1.6 x 1027 kg) is released 1 cm from the surface. What will be the speed of
the proton when it is 2 cm from the surface?
Fig. 22.40
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(20) 8. Consider two concentric hollow sphere as shown below. Use Gauss’s Law to find the equation for the electric
field in the region where c < r < d. There is charge +Q on the inner sphere and —Q on the outer sphere.
(You must start with Gauss’s Law and show each step clearly to obtain the answer.)
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