Physics 112
Exam 1
Spring 2015
1. Three identical negative point charges (Q = − 4.0 µC) are fixed in a vertical line, 10 cm apart.
What is the net ELECTRIC FORCE on the BOTTOM charge marked with a (*)?
A.
B.
C.
D.
E.
18 N down
22 N down
34 N up
16 N up
0N
Q
10 cm
Q
Q
10 cm
*
2. The top of a van de Graff is a spherical conducting shell (radius = 15 cm) with a net negative
charge of −20 µC.
Estimate the magnitude of the ELECTRIC FIELD 20 cm from the shell, as shown
A.
B.
C.
D.
E.
2.9 × 103
8.0 × 104
4.5 × 105
6.0 × 105
1.5 × 106
N/C
N/C
N/C
N/C
N/C
Shell, − 20 µC
×
20 cm
15 cm
3. Two POSITIVE particles of charge, + 16 µC and + 4 µC are some distance apart, as shown.
In which region or point (A-E), on a line connecting the charges, is E = 0?
(point C is mid-way between the charges)
C
A
B
+ 16 µC
D
E
+ 4 µC
Page 3 of 7
Physics 112
Exam 1
Spring 2015
4. Two point charges Q1 and Q2 are located inside a closed imaginary surface (area = A) and a third
point charge Q3 is outside the surface.
What is the total ELECTRIC FLUX through the surface?
(Q1, Q2 and Q3 are in Coulombs, A is in m2 and ε0 = 8.85 x 10-12 C2/Nm2)
A.
B.
C.
D.
E.
A/4
(
+Q − )/4
(
+ Q + )A/
(
+Q )/6
(
+Q )/
Q2
Q3
Q1
Surface, area = A
5. You do + 540 J of external work to bring three identical
POSITIVELY charged particles (+Q) from far away and arrange them
on the corners of an equilateral triangle, as shown.
How much external work is needed to arrange three NEGATIVELY
charged particles (−Q) in the same way?
(Q is the same magnitude in both cases)
A. 0 J
B. + 180 J
C. + 540 J
D. − 180 J
+Q
2 cm
2 cm
+Q 2 cm +Q
E. − 540 J
6. The figure shows several equipotential lines. A negative (−6.0 mC)
point charge has some initial speed at point H and moves to point K,
as shown. No external forces act on the charge during this process.
How much WORK does the electric force do on the point charge?
The electric force does
A. −36 mJ of work and the point charge slows down.
B. −54 mJ of work and the point charge slows down.
C. 0 J of work and the point charge moves at constant speed.
D. +54 mJ of work and the point charge speeds up.
E. +36 mJ of work and the point charge speeds up.
Page 4 of 7
Physics 112
Exam 1
Spring 2015
7. An ELECTRONVOLT is a unit of
A.
B.
C.
D.
E.
Electric potential
Electric field
Electric charge
energy
force
8. A 9.0 V battery is connected to the smallest faces of a rectangular block of germanium, with
dimensions, 5.0 mm × 1.0 mm × 1.0 mm. Germanium has a resistivity of 1.0 × 10-3 Ωm.
How much CURRENT is drawn from the battery?
(ignore the battery’s internal resistance and the resistance of the wires)
A.
B.
C.
D.
E.
0.29 A
0.65 A
1.8 A
3.2 A
7.2 A
5.0 mm
I
9. Calculate the POWER dissipated (transformed) by the 20 Ω resistor in this circuit.
A.
B.
C.
D.
E.
0.29 W
0.75 W
1.9 W
3.2 W
8.4 W
30 Ω
12 V
20 Ω
10. This exam is version A. Shade in circle A for Problem 10 on the bubble sheet.
Page 5 of 7
15 Ω
Physics 112
Exam 1
Spring 2015
11. You have a 9.0 V battery and a 24 V battery. Both have an internal resistance of 2.5 Ω. The two
batteries are connected to a 15 Ω resistor, as shown.
Calculate the TERMINAL VOLTAGE across the 9.0 V battery.
A.
B.
C.
D.
E.
7.6 V
8.5 V
9.0 V
9.8 V
10.9 V
9.0 V
24 V
2.5 Ω
2.5 Ω
15 Ω
12. A capacitor consists of two parallel metal plates and a 0.60 mm wide air-gap. Each plate has an
area of 76 cm2. The capacitor is connected to a 9.0 V battery and is fully charged.
What is the CHARGE in nC on the capacitor?
(1 nC = 10-9 C)
A. 0.74
B. 1.0
C. 350
D. 4200
E 6.6 x 10-4
13. The capacitor plates in the previous problem are moved slightly CLOSER together and no other
changes are made to the circuit.
What happens to the CHARGE and STORED ENERGY on the capacitor?
A. The charge and stored energy both DECREASE.
B. The charge and stored energy both INCREASE.
C. The charge INCREASES and stored energy DECREASES.
D. The charge DECREASES and stored energy INCREASES.
E. The charge and stored energy remain CONSTANT.
14. After the switch is connected, how long does it take for the
voltage across the 360 F capacitor to reach 85% of its
maximum final value?
A.
B.
C.
D.
E.
10 Ω
zero
0.5 seconds
12 minutes
1.9 hours
5.2 days
360 F
Page 6 of 7
Physics 112
Exam 1
Spring 2015
15. A 12 V battery is connected to three capacitors, as shown.
What is the EQUIVALENT CAPACITANCE of this circuit?
A. 4.4 µF
B. 6.0 µF
C. 8.3 µF
D. 17 µF
E. 20 µF
15.0 µF
12 V
6.0 µF
16. What is the VOLTAGE across the 6.0 µF capacitor in the above circuit?
A. 7.2 V
B. 6.1 V
C. 5.3 V
D. 4.4 V
E. 1.0 V
Page 7 of 7
4.0 µF
Physics 112
Exam 1
Answer key
Qu
1
2
3
4
5
A
E
D
E
C
6
7
8
9
10
A
D
C
D
A
11
12
13
14
15
E
B
B
D
B
16
A
Identifier (no credit)
Spring 2015