Download Sample final exam questions # 3

PHYS 121 SAMPLE FINAL EXAM 03
(answers on last page)
1. Two identical small charged spheres are hanging in equilibrium. They have equal masses of 0.03
kg as shown in the figure. The lengths of the strings are equal and the angle with the vertical is
identical and equal Θ=50. The distance between the charges is d =2.6 cm. The magnitude of the
charge on each sphere is closest to:
A. 4.40x10-8 C
B. 6.15x10-8 C
C. 2.74x10-7 C
D. 5.00x10-7 C
E. 9.45x10-6 C
2. Charges Q, -Q, and q are placed at the vertices of an equilateral triangle as shown. The total force
exerted on the charge q is:
A. toward charge Q
B. toward charge - Q
C. away from charge Q
D. at right angle to the line joining Q and -Q
E. parallel to the line joining Q and -Q
3. An electron traveling north enters a region where the electric field is uniform and points north. The electron:
A. speeds up
B. slows down
C. continues with the same speed in the same direction
D. moves with the same speed in the opposite direction
E. none of the above
4. Two identical charges of opposite sign are located on a horizontal axis. If a = 20 cm , d = 20 cm
and q = 8 nC, what is the electric field ( in unit-vector notation) at p on a vertical axis as shown in the
figure?
A. 1271 N/C i
B. -1271N/C j
C. 322 N/C i
D. 448N/C j
E. - 448 N/C i
5. An electric filed of magnitude 3500 N/C is applied along the +x-axis. Calculate the electric flux
through a rectangular plane 0.35 m wide and 0.7 m long if the plane is parallel to zy plane.
A. 1800Nm2/C
B. 860 Nm2/C
C. 420 Nm2/C
D. 200 Nm2/C
E. 0
6. A solid, insulating sphere of radius 8 cm has a total positive charge of 8 nC uniformly
distributed throughout its volume. Concentric with this sphere is an uncharged, hollow
conducting sphere whose inner radius is 20 cm and its outer radius is 25 cm. The surface
charge density on the inner surface of the conducting cavity is closest to
A. - 16 nC/m2
B. + 16 nC/m2
C. +65 nC/m2
D. - 450 nC/m2
E. +550 nC/m2
7. A 15-cm radius conducting sphere is charged until the electric field just outside its surface is 120 V/m. What is the
electric potential on the surface of the sphere, relative to the potential far away?
A. 5 V
B. 9 V
C. 18 V
D. 24 V
E. 36 V
8. A uniform electric field of magnitude 250 V/m is directed in positive x direction as shown.
What is the potential difference between two planes A and B perpendicular to the electric field
if the distance between the planes is 40 cm.
A. 100 V
B. -100 V
C. 80 V
D. -80 V
E. 0
9. What is the capacitance of air filled parallel-plate capacitor C if the plate area is 0.02 m2 and it must withstand
potential difference of 240 V without sparking across the plates? (the dielectric strength of air is E = 3x106 V/m)
A. 1.1 nF
B. 2.2 nF
C. 4.4 nF
D. 8.8 nF
E. 17.6 nF
10. A battery is used to charge a parallel-plate capacitor, after which it is disconnected. Then a dielectric with κ=2 is
inserted between the plates. This process will:
A. half the capacitance
B. half the potential difference between the plates
C. leave the capacitance unchanged
D. double the potential difference between the plates
E. double charge on each plate
11. Suppose that you want to design a heating coil that will convert electric energy to heat at a rate of 800 W for a
current of 4 A. If the coil is to be made of special Nichrome wire of resistivity 5x10-7Ωm and radius of 0.3 mm, the
length of the wire must be closest to:
A. 0.5 m
B. 3 m
C. 9 m
D. 28 m
E. 35 m
12. A wire of L = 62 cm length is placed in a uniform magnetic field of magnitude
B = 0.44 T. It carries a current of 2 A to the right. What are the magnitude and
direction of the force exerted on the wire by the magnetic field?
A. 0.15 N, up
B. 0. 32 N, down
C. 0. 41 N, down
D. 0.54 N, up
E. 0.54 N, down
An electron moving with a speed of 1.87x107 m/s along the +x direction enters a region between two parallel plates
separated by d = 20 mm with a potential difference of V = 100 V between them. The direction of velocity is perpendicular
to the electric field which points along the y direction.
13. What is the magnitude of a uniform magnetic field, applied perpendicular to both the electron path and the electric
field, that will allow the electron to travel in a straight line?
A. 6.7 x10-5 T
B. 8.9 x10-4 T
C. 2.67 x10-4 T
D. 5.0x10-3 T
E. 0.8x10-3 T
14. Through what potential difference might the electron have been accelerated through before entering the region with
the E and B fields?
A. 1000 V
B. 2000 V
C. 3000 V
D. 4000 V
E. 5000 V
15. The two semicircular arcs AD and HJ of radii R1 =5 cm and R2 = 8 cm respectively,
form part of the circuit carrying current of I = 2 A. Find the magnetic field produced by
those arcs of current at point C – the center of curvature.
A. 81.0 µT
B. 44.2 µT
C. 12.8 µT
D. 7.2 µT
E. 4.7 µT
The circuit shown at the right is in a uniform magnetic field that is into the page
and decreasing in magnitude at the rate 150 T/s.
16. What is the magnitude of the induced electromotive force in the circuit?
A.
B.
C.
D.
E.
1.51 V
2.16 V
3.45 V
4.20 V
5.75 V
17. What is the net current flowing in the circuit?
A. 0.18 A
B. 0.22 A
C. 0.40 A
D. 0.62 A
E. 1.5A
18. An LC circuit is shown in the figure below. The 9 nF capacitor is initially charged by
the 12 V battery when S is in position a. Then switch S is thrown to position b so that the
capacitor becomes connected across the 2.81 mH inductor. The maximum value of current in
the oscillating circuit is:
A. 0.01 A
B. 0.02 A
C. 0.03 A
D. 0.04 A
E. 0.05 A
19. When a particular inductor is connected to a sinusoidal voltage with a 120 V amplitude
(switch is closed), a peak current of 3 A appears in the inductor. What is the maximum current
in the circuit if the frequency of the applied voltage is doubled?
A. 0.5 A
B. 1.5 A
C. 3.0 A
D. 4.5 A
E. 6.0 A
Questions 20 and 21 use the following: An LCR circuit consists of a 50-Ω resistor, a 2-µF
capacitor, and 12-mH inductor, connected to a generator with maximum voltage of 210 V
operating at 500 Hz.
20. What is the magnitude of the angle between the current in the circuit and the voltage
of the generator?
A. - 67.60
B. + 53.60
C. - 47.10
D. - 32.60
E. -+17.60
21. Find the average power dissipated in the resistor?
A. 25 W
B. 44 W
C. 65 W
D. 80 W
E. 150 W
22. For the circuit shown at right, which resistor has the largest
voltage drop?
A. R1
B. R2
C. R3
D. R4
E. R3 and R4 together
23. In the series LCR circuit shown at right, the resistance R = 200 Ω, the inductance L =
0.005 H, and the capacitance C = 0.3 µF. An AC generator supplies an alternating emf given
by E(t) = 5sin(50000t). What is root mean square voltage across the capacitor in the circuit?
A. 0.15 V
B. 0.88 V
C. 1.76 V
D. 2. 45 V
E. 5. 00 V
24. When switch S is open in the circuit shown the ammeter reads 2.0 A. When S is
closed, the ammeter reading will:
A.
B.
C.
D.
E.
increase slightly
remain the same
decrease slightly
double
halve
25. How long would it take to charge the capacitor in the circuit shown up to 80% of the
battery voltage, if the capacitor initially has no charge? The resistance R = 150 Ω and the
capacitance, C = 10 µF.
A. 1.0 ms
B. 1.5 ms
C. 2.0 ms
D. 2.4 ms
E. 3.0 ms
26. A 10-microfarad capacitor has been charged to a potential of 150 volts. A resistor of 25 ohms is then connected
across the capacitor through a switch. When the switch is closed for 20 time constants, the energy dissipated in the resistor
(energy lost) is most nearly:
A.
B.
C.
D.
E.
1x10-7J
1.1x10-1J
9x101J
9x103 J
1x104J
27. The magnitude of the force on a particle of charge q placed in the empty space between two infinite parallel plates
with a spacing d and a potential difference V is proportional to:
A. qV/d2
B. qV/d
C. qv2/d
D. q2V/d
E. none of the above
28. If the connecting wires and the battery in the circuit shown have negligible
resistance, the current through the 6-ohms resistor is most nearly:
A. 1/3 A
B. 1/2 A
C. 1 A
D. 3/2 A
E. 2 A
29. The expression for the current in 5-ohm resistor of the circuit shown for time t greater
than zero is:
A. 2e-2t
B. 2e-5t
C. 2(1-e-2t)
D. 2(1-e-2.5t)
E. 4e-2t
30. Which of the following sketches shows the
ratio of root-mean-square (rms) voltage to rms
current for a capacitor as a function of the applied
frequency f ?
31. Faraday's law tells us that the EMF induced in a coil of wire by an external magnetic field is proportional to:
A)
B)
C)
D)
E)
the rate of change of the magnetic field
the magnetic flux through the coil
the rate of change of the magnetic flux
the velocity of the coil through the field
none of these
32. In the sketch at right, the current in the long wire is flowing to
the right and is increasing. The induced current in the loop will
flow:
A)
B)
C)
D)
E)
clockwise
counterclockwise
zero
downward on both left and right sides of the loop
upward on both left and right sides of the loop
i
33. A 100-turn coil has an inductance of 3.5 H. At a moment when the current in the solenoid is changing at 20 A/s, the
induced emf in it is:
A)
B)
C)
D)
E)
0 V.
0.70 V.
7.0 V.
70 V.
7000 V.
34. The current in an RL circuit builds up to 75% of its steady-state value in 10.0 s. The inductive time constant is:
A)
B)
C)
D)
E)
3.6 s.
7.2 s.
14.4 s.
34.8 s.
unknown - need to know R and L.
35. Choose the entries for the blanks that make the following statement about AC circuits correct:
Across a capacitor, the peak voltage _____ the peak current by /2 radians. Across an inductor, the peak voltage
______ the peak current, and across a resistor the peak voltage _________ the peak current.
A)
B)
C)
D)
E)
leads, lags , is in phase with
lags, leads, is in phase with.
leads, is in phase with, lags.
lags, lags, leads
exceeds, follows, leads.
Answers to Physics 121 Sample Final Exam 03
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B