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Unformatted text preview: Physics 272— Practice Exam ll
Spring 2010
Prof. Mohan Kalelkar Your name sticker with exam code . The exam will last from 3:25pm to 4:30pm. Use a # 2 pencil to make
entries on the answer sheet. Enter the following id information now,
before the exam starts. . In the section labelled NAME, enter your last name, then ﬁll in the'
empty circle for a blank, then enter your ﬁrst name, another blank,
and ﬁnally your middle initial. . Under STUDENT # enter your 9~digit Student ID Number. Under
COURSE enter 272. Under CODE enter the exam code given above. . During the exam, you may use pencils, a calculator, and ONE 8%” X 11” sheet of paper with formulas and notes. ' . There are 16 multiple~choice questions on the exam. For each ques—
tion, mark only one answer on the answer sheet. There is no subtrac
tion of points for an incorrect answer, so even if you cannot work out
the answer to a question, you should make an educated guess. At the
end of the exam, hand in only the answer sheet. Retain this question
paper for future reference and study. . Useful numerical constants are given on the next page. Before starting
the exam, make sure that your copy contains the page of constants
and all 16 questions. Bring your exam to the proctor if this is not the
case. Acceleration due to gravity g = 9.8 772/32
Elementary charge 6 =2 1.6 x 10‘“19 C Proton charge :2 1.6 x 10“19 0 Electron charge :2 —1.6 x 10“19 C 1 electron volt (3V) :2 1.6 x 10‘“19 J Proton mass = 1.673 X 10“27 kg 2 938.3 MeV/c2
Electron mass =2 9.11 x 10"31 kg = 0.511 M'eV/c2
1/47reo = 9 x 109 N  7122/02 M0 = 47r x 10“7 Tm/A Powers of ten: femto(f) pico(p)
1015 10*12 centi(c) kilo(k) 10—2 10+3 microm)
10“6
Giga(G) 1. Consider a closed Gaussian surface. Which of the following are true? a I: If the electric ﬁeld points inward everywhere on the surface, the net
enclosed charge must be negative. 0 II: If the surface encloses a positive +Q and a negative charge ~3Q,
the total electric flux through the surface must be negative. 0 III: If the total electric flux through the surface is zero, then the
electric ﬁeld must be zero everywhere on the surface. a) All three statements are true
b) I and II are true; III is false
c) I is true; II and III are false
d) II is true; I and III are false
8) II and III are true; I is false 2. See the ﬁgure for this question. A long, insulating solid cylinder of radius
R carries a uniform volume charge density p. What is the magnitude of the
electric ﬁeld for 7“ > R, where 7" is the perpendicular distance from the axis of the cylinder?
a) pr/Zeo ﬂ ‘
b) pRg/Zeor } R
c) p’F/360
d) pR3/360T2
e) None of the other answers ’ ' ’ 3. See the ﬁgure for this question. Consider two concentric, conducting spher—
ical shells. The outer shell has a charge of +3,LLO on its inner surface, and a
charge of ~7MC on its outer surface. What are the charges on the surfaces
of the inner shell with radii R1 and R2 respectively? a) Zero and ~3MO b) Zero and +4/.LC' c) 3/.LC' and +3/.LC' d) +3/.LC' and 3/.LC'
) None of the other answers (D 4. A charge of +5710 is on the y—axis at y = +60m, and a charge of —5nC
is on the y~axis at y = —6cm. What is the electric potential at the point
(8,0), i.e. X = +80m, y = 0? a) 450 V
b) 540 V
c) Zero
d) 9000 V
e) 900 V 5. An electron initially at rest is accelerated through a potential difference of
3 V. What speed does it acquire? a) About 1 x 102 m/s
b) About 1 x 103 m/s
c) About 1 X 104 m/s
d) About 1 X 105 m/s
'e) About 1 X 106 m/s 6. A charge Q is at the origin. Concentric with it is a conducting spherical
shell of inner radius a and outer radius 17. The shell has no NET charge.
The radial distance 7" is measured from the center. In the region r < a,
What is the electric potential V(r), if we make the usual choice that V = O
at inﬁnity? ’ a) Q/47reor b) (Q/47reoa) +>(Q/47reor) c) (Q/47reob) + (Q/47reor) d) (Q/47reor) — (Q/47reoa) e) (Q/47reob) + (Q/47reor) — (Q/47reoa) 7. Two conducting hollow spheres have different radii. A thin conducting Wire
connects the spheres. A charge Q is placed on the smaller sphere. What
Will then happen? a) Charge Will flow until the spheres have equal potentials b) Charge Will flow until the spheres have equal charges c) All of the charge will flow to the larger sphere d) Charge Will flow until the spheres have equal electric ﬁelds at their
surfaces e) There Will be no ﬂow of charge, so the charge Q will remain on the
smaller sphere 8. A parallel—plate capacitor is connected to an adjustable power supply. Which
of the following changes will cause the capacitor’s capacitance to increase? 0 I: Increasing the charge put out by the power supply
0 II: Decreasing the voltage output of the power supply 0 III: Decreasing the separation between the plates a) All three will increase the capacitance b) I and II, but not III c) I and III, but not II d) III only e) None of these will increase the capacitance 9. Capacitors A and B have capacitances of 3 HF and 5 “F respectively. They
are connected in series to a 200 V battery. What will be the ﬁnal charges
on capacitors A and B respectively? a) 600 HO and 1000 #0
b) 375 HO and 375 MC
c) 1600 MC and 1600 MC
d) 1000 MC and 600 MC ) e 107 MC and 107 MC 10. Two capacitors have capacitances of 7 MF and 5 NF respectively. They are
connected in parallel to a battery. After the battery has charged them, the
7 HF capacitor has a stored energy of 2.24 x 10‘4J. What is the charge on
the OTHER capacitor, i.e. the 5 uF one? ) 40 p0 a b) None of the other answers
c) 96 MC d) 56 MC e) 47 MC 11. Two straight wires A and B of circular cross—section are made of the same
metal and have equal lengths, but the resistance of wire A is four times
that of wire B. How do their radii compare? 0"
v
3 D>

i
U21
\
“k 12. 13. 14. 15. Wires A and B are made of the same metal, but the radius of wire A is
twice that of wire B. The current in wire A is four times that in wire B.
How are the drift speeds of charge carriers in the two wires related? a) ’UA = 8’03
b) ’UA = ’03
C) ’UA = 2'03
Cl) ’UA = 161213
e) 12A = 4113 A toaster of resistance 10 Q is connected to a 120 V dc source. What will
be the approximate cost of operating the toaster for 2 minutes, if electricity
in New Jersey costs 17 cents per kilowatthour? a) About 0.8 cent
b About 1.2 cents
About 1.6cents « l
C)
d) About 1.8 cents
e) More than 1.9 cents Two resistors have resistances R and BB respectively. They are connected
in series to a 12 V battery. If the current in the resistors is 2.0 A, what is
the value of R? a) 3.0 Q wows c) 2.0 Q d) 6.0 S) e) None of the other answers Resistors R1 and R2 are connected in parallel to an 18 V battery. The two
resistors have an equivalent resistance of 5 Q. If the current in resistor R2
is 1.5 A, in which of the following ranges does the value of R1 lie, in units
of ohms? a) R1<4
m4gm<8
)sgm<m
) ) e 16. See the ﬁgure for this question. Note that two of the resistors have the
same resistance R, While the one on the far right has resistance 2B. The
current through the middle resistor is 2.0 A down. What is the value of R? a) 6.09
b) 5.09
c) 15.09
d) 4.09 e) 100 9 ...
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 Spring '10
 Brahmia
 Physics, Electron, Charge, Electric charge

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