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Instructor: P. Kumar
PHYSICS DEPARTMENT
PHY 2049
Exam III
July 16, 2008
Signature:
Name (print, last first):
On my honor, I have neither given nor received unauthorized aid on this examination.
YOUR TEST NUMBER IS THE 5-DIGIT NUMBER AT THE TOP OF EACH PAGE.
DIRECTIONS
(1) Code your test number on your green answer sheet (use 76–80 for the 5-digit number). Code your name
on your answer sheet. Darken circles completely (errors can occur if too light). Code your student number on
your answer sheet.
(2) Blacken the circle of your intended answer completely, using a number 2 pencil. Do not make any stray
marks or the answer sheet may not read properly.
(3) The answers are rounded off. Choose the closest to exact. There is no penalty for guessing.
>>>>>>>>WHEN YOU FINISH <<<<<<<<
Hand in the green answer sheet separately.
1. In the circuit shown, L = 56 mH, R = 4.6Ω and V = 12.0 V. The switch S has
been open for a long time then is suddenly closed at t = 0. At what value of t
(in msec) will the current in the inductor reach 1.1 A?
L
R
R
(1) 6.67
(2) 10.5
(3) 2.88
(4) 19.0
V
S
(5) None of these
2. Refer to the previous problem. What is the total energy stored in the inductor a long time after the switch is closed?
(1) 0.19 J
(2) 0.048 J
(3) 0.76 J
(4) 0.034 J
(5) None of these
3. A real inductor has resistance because it is composed of a coil of wire with nonzero resistivity. To measure the inductance
of a coil, a student places it across a 12.0-V battery and measures a current of 0.72 A. The student then connects the
coil to a 30.0-V (rms) 60 Hz generator and measures an rms current of 0.86 A. What is the inductance of the coil?
(1) 0.081 H
(2) 0.51 H
(3) 34.9 H
(4) 30.6 H
(5) None of these
4. Sunlight of intensity 1340 W/m2 is incident normally on a 9 km × 9 km rectangular sail of negligible mass attached to
a spaceship of mass 12,000 kg. If the sail is perfectly black, how long will it take the spaceship to move 1 km starting
from rest?
(1) None of these
(2) 180 sec
(3) 8.1 sec
(4) 24 min
(5) 4.8 hr
5. A dimmer for a stage light at a theater consists of a variable inductor, whose inductance is adjustable between zero and
Lmax , connected in series with a lightbulb. The electrical supply is 120 V (rms) at 60.0 Hz; the lightbulb is rated at
1.0 kW for this voltage. What Lmax is required if you want the rate of energy dissipation in the lightbulb to be variable
by a factor of 10 from its upper limit of 1.0 kW? Assume that the resistance of the lightbulb does not depend on its
temperature.
(1) 0.11 H
(2) 0.45 H
(3) 1.2 H
(4) 65 mH
(5) 21 mH
6. A small light bulb is placed 3 cm inside a solid cube of Lucite (n=1.50) and turned on. What is the radius (in cm) of
the illuminated circle that would be seen by an observer standing above the Lucite block?
(1) 2.68
(2) 3.00
(3) 3.35
(4) 4.50
(5) 3.42
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7. A horizontal beam of unpolarized light is incident upon a stack of 4 polarizers with axes of polarization, in order and
measured clockwise from the vertical, at 30◦ , 75◦ , 120◦ and 180◦ . What is the ratio of the intensity of the transmitted
beam to that of the incident beam?
(1) 0.031
(2) 0.063
(3) 0.023
(4) 0.047
(5) 0.0063
8. An oscillating LC circuit consists of a 2 mH inductive coil and a 4µF capacitor. The capacitor has a potential difference
of 0.75 V when the current through the coil is 30 mA. What is the maximum possible charge on the capacitor [or current
through the inductor]?
(1) 4 × 10−6 C
(2) 3 × 10−6 C
(3) 2 × 10−6 C
(4) 8 × 10−9 C
(5) 0.75 C
9. The average intensity of light from an incandescent light bulb is 300 mW/m2 on a particular surface. Assuming that
the light is in the form of an electromagnetic plane wave, what is the maximum magnetic field amplitude, Bm ?
(1) 5 × 10−8 T
(2) 7 × 10−8 T
(3) 3.5 × 10−8 T
(4) 21 T
(5) 15 T
10. Light traveling horizontally enters a right prism through the hypotenuse, as
shown in the figure. The index of refraction of the prism is n = 1.6. At what
angle is the light deflected from horizontal?
(1) 36◦
(2) 26◦
(3) 19◦
(4) 45◦
(5) 31◦
11. An arrangement for generating a traveling electromagnetic wave in the shortwave radio region of the spectrum works
as follows: an LC oscillator produces a sinusoidal current in the antenna, which generates the wave, traveling outward
at the speed of light. What is the wavelength (in meters) of the wave emitted by this system if L = 0.323µH and
C = 45.0pF?
(1) 7.19 m
(2) 1.14 m
(3) 719 m
(4) 114 m
(5) None of these
12. A current of 1A is used to charge a parallel plate capacitor with square plates. If the area of each plate is 0.6m2 the
displacement current through a 0.3m2 area wholly between the capacitor plates and parallel to them is:
(1) 0.5A
(2) 1A
(3) 2A
(4) 0.7A
(5) 0.25A
THE FOLLOWING QUESTIONS, NUMBERED IN THE ORDER OF THEIR APPEARANCE ON THE ABOVE
LIST, HAVE BEEN FLAGGED AS CONTINUATION QUESTIONS: 2