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Physics 124B Spring 2007 Exam 2
Time Allowed: 2 hours
Total points: 105
Some Physics Constants: electronic charge
Coulomb constant
Permittivity of free space
Mass of electron
Mass of proton
Name:
e = 1.6×10-19 C
k = 9.0×109 Nm2/C2
ε0 = 8.85×10-12 C2/Nm2
me = 9.11×10-31 kg
mp = 1.67×10-27 kg
A Yes/No questions (Total points=10. Points per question=2)
1. The electric field is zero at the mid-point of an electric dipole.
Yes
No
2. When a positive charge moves from a place of high potential to one of lower potential, its kinetic energy
increases under the influence of electric force alone.
Yes
No
3. The electrical resistance of a metallic wire increases with its thickness
Yes
No
4. When a dielectric material is inserted between the charged plates of a parallel plate capacitor, the
potential difference increases if the plates are isolated.
Yes
No
5. The diagram shows a circuit connected to a 120V AC power supply with a circuit breaker that trips
when the current exceeds 10 A. A 650 W toaster-oven and a 500W hair dryer are connected as loads A
and B. If a 75W light bulb is now connected as load C, will the circuit trip?
Yes
No
B Multiple Choice Questions (Total points =60 points. Points per question = 5)
6. Three resistors 5, 6, 7 are connected in parallel. Find the equivalent resistance.
(a) 0.14
(b) 1.50
(c) 1.96
(d) 18 
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7. The emf of a battery is 2.0V and its internal resistance is 1.0  . What is the potential difference
between its terminals when it is connected to a load of resistance 5.0  ?
(a) 2.0 V
(b) 0.33 V
(c) 1.12 V
(d) 1.67 V
8. The potential difference between two infinitely large parallel metal plates 5.0cm apart is 400V . What is
the electric field between the plates?
(a) 800 N / C
(b) 4000 N / C
(c) 8000 N / C
(d) 40000 N / C
9. Refer to the previous question. If an electron is introduced at rest at the plate with the lower potential,
what is its speed when it reaches the other plate?
(a) 2.7  10 5 m / s
(b) 3.5  10 6 m / s
(c) 1.2  10 7 m / s
(d) it cannot reach the plate with higher potential
10. Two resistors with resistances R A and R B are connected in series. Let VA , VB denote the potential
differences across each, and I A , I B the current passing through each. Which of the following statements
is correct if R A  RB ?
(a) VA  VB
(b) V A  VB
(c) I A  I B
(d) I A  I B
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11. The energy stored in a capacitor is 10.0 J . The capacitance is 50nF . What is the potential difference?
(a)
(b)
(c)
(d)
20kV
40kV
60kV
80kV
12. The diagram shows a charged rod brought near an electrically isolated metallic sphere. Let
V A , V B , V C denote the electric potentials of the points A,B and C as indicated. Which of the following
statements is necessarily true ?
A
B
B
C
(a) VB  VC
(b) VB  VC
(c) V A  VB
(d) None of the above, because it depends on whether the charge on the rod is positively or negative
13. The potential at a point A is 100V . It is in a region where the electric field is uniform with the value
500 V / m pointing east. What is the potential at the point B in the same region that is 3m directly north
of the point A?
(a) 0V
(b) 100V
(c) 1500V
(d)  1500V
14. When a resistor is connected to an ideal battery with emf equal to 2V , the current it draws is 40mA .
What is its resistance?
(a)
(b)
(c)
(d)
50
5
0.5
0.05
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15. The one-sided area of each plate in a parallel plate capacitor is 0.4m 2 . The distance between the plates
is 1mm . When a potential of 250V exist between the plates, how much charge exists on the positive
plate?
(a) 3.54  10 9 C
(b) 2.93  10 8 C
(c) 8.85  10 7 C
(d) 9.12  10 6 C
16. A magnesium ion Mg++ creates positive electric potential in its environment. At what distance from the
ion is the electric potential equal to 75V?
(a)
(b)
(c)
(d)
5.12  10 11 m
3.84  10 11 m
6.20  10 6 m
2.78  10 6 m
17. The cross-section of a rail used as railroad track is a 3cm  5cm . How long should the rail be for its
resistance to be 5 ? ( The resistivity of iron is 9.7  10 8   m )
(a)
(b)
(c)
(d)
77km
54km
23km
19km
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C Multi-step problems (Total points=35)
18. The figure below shows an electric dipole of point charges  4nC and  4nC separated by a distance
of 2m . The points A and B are at distances of 1m and 2m from the positive charge.
A
-4nC
B
+4nC
(a) (5 points) Find the electric potentials at A and B.
(b) ( 5 points) A bare nucleus of lithium (atomic number Z=3, atomic mass A =7 ) is introduced at
rest a the point A. How many eV is its kinetic energy when it reaches the point B?
(c) ( 3 points) What is the speed of the lithium ion at the point B?
(d) .(2 points) Sketch the equi-potential surface for zero potential on the figure
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(e) (BONUS: 5 points) Determine the magnitude and direction of the electric field at the point A.
19. In the circuit below, the values of the resistances are given by R1  6 R2  3 R3  2 R4  1
and the emf of the ideal battery is 12V.
R1
B
R3
R2
C
A
R4
(1) (5 points) Find the equivalent resistance across the terminals of the battery.
(2) (3 points) Find the current flowing through the resistance R4
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(3) (2 points) Find the potential difference VB- VA
(4) (5 points) Find the power dissipated in R1
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