A charged sphere with an initial speed has a final speed when accelerated through a potential difference
. What potential difference is needed to accelerate the same charged sphere from an initial speed to a final
spee
...
A charged sphere with an initial speed has a final speed when accelerated through a potential difference
. What potential difference is needed to accelerate the same charged sphere from an initial speed to a final
speed ?
(A)
(B)
(C)
(D)
(E)
Answer C
Correct. Using conservation of energy in the first situation yields
and in the second situation yields
The drop in potential energy in the second situation is four times the drop in potential energy in the first
situation, so the potential difference drop in the second situation must be four times larger than that in the
first situation. Hence, C is the answer.
Two small spheres have charges of equal magnitude and opposite signs. The spheres are initially at rest and separated
by a very large distance, as shown in the initial configuration. The sphere on the right is then moved at constant speed to
another position closer to the charge on the left, as shown in the final configuration. Assume the spheres are small
compared to both the initial and final separations.
2. Which of the following graphs shows the electric potential energy for the two spheres as a function of the
distance between the two spheres?
AP PHYSICS C: ELECTRICITY AND MAGNETISM Scoring Guide
mcq
AP Physics C: Electricity and Magnetism Page 1 of 94(A)
(B)
(C)
(D)
Scoring Guide
mcq
Page 2 of 94 AP Physics C: Electricity and Magnetism(E)
Answer B
Correct. This selection uses the equation for electric potential energy, . As the distance
between the spheres decreases, the absolute value of the potential energy increases. Also, since the two
charges have opposite signs, the potential energy is negative.
3. The work done by an external force to move the sphere of charge from the initial configuration to the final
configuration is . The sphere with charge of is then replaced with an identical sphere charge of . This
new sphere is moved from the initial configuration to the final configuration shown. Which of the following
expressions is correct for the work done by the external force to move the sphere with charge , in terms of
?
(A)
(B)
(C)
(D)
(E)
Answer A
Correct. This selection uses the equation for the work done by an external force. The work done to bring
the two original spheres from the initial to final configuration is given by
. For the new spheres, is
not ; thus, the work done is given by
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