Spring 2015 - Physics 162 - Exam 3 Review
1. Suppose you have two point charges of opposite sign. As you move them
farther and farther apart, the potential energy of this system relative to
infinity
(a) increases.
(b) decreases.
(c) stays the same.
2. The graph in the figure shows the variation of the electric potential V (x)
(in arbitrary units) as a function of the position x (also in arbitrary units).
Which of the choices below correctly describes the orientation of the xcomponent of the electric field along the x-axis?
?
(a) Ex is positive from x =
2 to x = 2.
(b) Ex is positive from x =
x = 2.
2 to x = 0, and negative from x = 0 to
(c) Ex is negative from x =
x = 2.
2 to x = 0, and positive from x = 0 to
(d) Ex is negative from x =
2 to x = 2.
3. A narrow copper wire of length L and radius b is attached to a wide copper
wire of length L and radius 2b, forming one long wire of length 2L. This
long wire is attached to a battery, and a current is flowing through it. If
the electric field in the narrow wire is E, the electric field in the wide wire
is
(a) E.
(b) 2E.
(c) 4E.
(d) E/2.
(e) E/4.
4. A -2.0 nC charge and a +2.0 nC charge are located on the x-axis at
x = 1.0 cm and x = +1.0 cm, respectively.
(a) Other than at infinity, is there a position or positions on the x-axis
where the electric field is zero? If so, where?
(b) Other than at infinity, at what position or positions on the x-axis is
the electric potential zero?
5. Two 5.0 mm ⇥ 5.0 mm electrodes with a 0.10-mm-thick sheet of Mylar
( = 3.1) between them are attached to a 9.0 V battery. Without disconnecting the battery, the Mylar is withdrawn. (Very small spacers keep the
electrode separation unchanged.) What are the charge, potential di↵erence, and electric field (a) before and (b) after the Mylar is withdrawn?
6. Pencil “lead” is actually carbon (⇢ = 3.5 ⇥ 10 5 ⌦m). What is the current
if a 9.0 V potential di↵erence is applied between the ends of a 0.70-mmdiameter, 6.0-cm-long lead from a mechanical pencil?
Answers:
1. A
2. B
3. E
4. (a) no
(b) x = 0 m
5. (a) C0 = 2.21 ⇥ 10 12 F
C = 6.86 ⇥ 10 12 F
i. Q = 6.17 ⇥ 10 11 C
ii. Vc = 9.0 V
iii. E = 29, 000 V/m
(b)
i. Q0 = 2.0 ⇥ 10 11 C
ii. Vc = 9.0 V
iii. E0 = 90, 000 V/m
6. R = 5.5 ⌦, I = 1.6A
Full solutions are posted below.