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Unformatted text preview: 301,0 ”TIQNS Physics 272— Exam ll
Wednesday, March 24, 2010
Prof. Mohan Kalelkar Your name sticker with exam code . The exam will last from 3:25pm to 4:25pm. 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 9 = 9.8 m/s2
Elementary charge a = 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 (6V) = 1.6 x 10‘19 J Proton mass = 1.673 x 10’27 kg = 938.3 MeV/c2
Electron mass = 9.11 x 10‘31 kg = 0.511 MeV/c2
U®w0= gxuﬁthWCZ ,LLO = 471’ ><10‘7 Tm/A Powers of ten: fernto(f) pico(p) nano(n) micro([1) milli(m)
10—15 10“12 109 10—6 10*3
centi(c) kilo(k) Mega(M) Giga(G) Tera(T) 102 10+3 10+6 10+9 10+12 ,pz/gs “Flo/v 7 QUEST/WU ‘Z auEST/oA/S ”13/5. 1. A point charge is placed at the center of a spherical Gaussian surface.
Which of the following will cause the total electric flux through the surface
to change? 0 I: Moving the charge off center, but still inside the sphere
0 II: Increasing the radius of the sphere o III: Replacing the sphere by a cube of the SAME volume Cb =1 "EL garrcwséD
D a) I will change the flux, but not II or III b) II will change the flux, but not I or III a {2 K M c) II and III will change the flux, but not I 2 d) I and II will change the flux, but not III 4957.94 764,2» W
g e) None of these will change the flux 2. See the ﬁgure for this question. A charge of +562 1s at the origin. Concentric
with it 1s a conducting spherical shell of 1nner radius R1 and outer radius R2.
The shell is given a NET charge of —2Q. The radial distance 7" is measured
from the origin. In the region 7" < R1, the magnitude of the electric ﬁeld is 1/47reotimes g; M h< K]
a) Zero
—>b) 5Q/T2 W MW +54?
0; ~2Q/r2 g) s 8 (MM, 7; 22.55?
) 3Q/7"2
None of the other answers 6 / ﬂ .
g .2 47’ £6 A3 3. In the preceding problem, what 1s the charge on the surface of the shell at M2? aha; =% g m a. w a) Zero . E g 2 “F62
b) 262 45 W ”W
e) ~5Q Ad 1,142.91 45% «ﬂange.
«w? d) +362
e) None of the other answers ” 2Q "’ CH fﬂ>9 + 3 Q . 4. A charge of +4 710 is located at the point (20 0771,15 cm), and another
charge of +4 710 is located at (20 cm, —15 cm). What is the electric po
tential at the origin, assuming it is taken to be zero at inﬁnity? a) About 230 V [29); ,7!— :19); :12? 6M 1
b) Zero
*9 c) About 288 V
d) About 360 V 612‘ / 0‘, x I °
e) About 173 V 2;. $288K 5. An object of mass m1 carries a charge Q. An object of mass m2 carries
a charge —Q. The two are placed a distance d apart, and then released
from rest. Of course they move towards each other. When the distance
between them is d/ 3, let M and v2 represent their respective speeds. Which
of the following equations is a correct application of the principle of energy
conservation to this situation? (Neglect any effects due to gravity). a) Q2/47T60d= 3Q2/4we0d + 2771101 2+ 27712712? b) Q2/47reodz —3Q22/47T0d + ~2mlv1 ~l—2 277121;; c) Q2/47reod + 277112112: 3622/47r50d + 277121713 d) —Q2/47reod + 27711711 2 —3Q2/47reo d 2+ 774—27122
”963) —Q2/47r50d = —3Q2/47T60 d + 27711711 + 377127;; 6. A hollow, insulating sphere of radius a has a charge Q distributed uniformly
over its surface. The radial distance 7" is measured from the sphere’s center.
What is the electric potential for r < a, if we make the usual chgce that it
is zero at inﬁnity? ‘ n8 3 ﬂ 1
Q £2): 167%,, ”a. 113 is? 1:; 8717mm :1:
47m?"
0) Q/47re:a+Q/47reor V” 'T 5“ an: An“. {he11:00;
d; Q/47r60a — Q/47T607‘ 1 & all Q
Zr ,_ —— ..—. ._______.... ..—— .
8 9° ~+W507F «02,2120 at» 7. See the ﬁgure for this question. Two charged, conducting spheres of un—
equal radii are connected by a thin, conducting wire. When equilibrium is
reached, which of the following is true? a) The spheres will have equal electric ﬁeld magnitudes at their surfaces .4
b) The larger sphere will be at a higher electric potential I
c) The larger sphere will have a greater electric ﬁeld magnitude at its M surface .__,.~,.’ &I ~32,” .83. w‘? d) The larger sphere will have more charge on it [1.7720 3""? 57 #fE‘P K 2
e) The spheres Will have equal electr1c charges Kl =:> ‘9 ' ...
A2, H R2
8. A capacitor of capacitance C' 1s charged by a battery applying a potential
difference of 8 V. The capacitor is then disconnected from this battery, and
attached to a different battery that applies a potential difference of 16 V. What will be the new capacitance of the capacitor?
~9 a) 0 W W w‘}
b) 40 f
c) 0/4 ,0 M ,
d) 20
) e 0/2 9. See the ﬁgure for this question. Three capacitors, each of capacitance C,
are connected as shown. What is the equivalent capacitance of the combi— nation? ,.
a00/3 PW: C12 5'" 5+6 / 26' _ 1 3
”513) 20/3 .1 ’_ +._..;_J—+w—,~5~—"
(13:3 ICEsz ”— cm 5 2° 0 26
302 1 =3;
e) / «are» 39 10. A parallelplate capacitor is charged by a battery, and then disconnected
from it. If the plate separation is then doubled, what happens to the po—
tential difference V between the plates and the energy U stored in the
capacitor? & 0L 6) 5L .
‘5', M r" a
v9 1:) V gets doubled; U gets doubled V C '/ 6, #70; 6’, ﬂ
3, Fl ) V gets doubled; U stays the same 0) V gets doubled; U gets halved " 'QV“ "'
)
) A ' .ao Q —..
V gets halved; U gets halved C 2 6y A V gets halved; U stays the same Q W W; i W e 11. Two wires are both made of copper and have the same lengths, but one
wire is thicker (i.e. has a greater diameter) than the other. How are their resistivities and resistances related?
' . . . 237% W.) M a) Same resistance; thicker wire has more res1st1v1ty ) Same resistance; thicker wire has less resistivity W J7 *
c) Same resistance; same resistivity L.
> d) Same resistivity; thicker wire has less resistance R ‘3 f 7r (
e) Same resistivity; thicker wire has more resistance 12. A light bulb is marked “75 W, 120 V” which means that it puts out 75 W
of power if connected to a 120 V source (DC). If this bulb is connected to
a different source (DC) such that it draws a current of 0.7 A, what Will be
its power output? (Assume that the resistance of the bulb stays the same). 2 2
a) About 107W L g ., (1250),. ., 2 52
b About52W P3 R, ﬂ —' M I4 .
About 94 W ) C) d) Abou 75W ‘7 1??
> t WWW: ”’7; >2 C ) e About84W
:2 w M. Q 13. Resistors R1 and R2 are connected in series to a 16 V battery. The two
resistors have an equivalent resistance of 8.0 Q. If the voltage drop across
R2 is 15 V, what is the value of the other resistance R1? a) 7.59 E'sj,“ =5 u—Ié—vz; 7,014.
b) 8.5 9 “an X. 9
—_; c) 0.5 Q :3 V I __
d) 1209 K2 Ea "3 32"" 7'55?”
e) None of the other answers “a g_ p .. '7. 5 .5: 0  5'5)“. 14. See the ﬁgure for this question The batteries have negligible internal resis—
tance. What is the current in resistor R1. a) 11.0Afromatob W I £74m Ali/{E A». TMW
~92; ligﬁiiﬁﬁilﬁﬁ U’W W 110+: a 7
d) 18.0Afromatob 9 I ”4» a ,4) NW0] :9
e) 18.0Afrombtoa‘ j: Z: 15. In the preCeding problem, what is the value of the resistance R2? Hint: it
is possible to do this without using the result of the previous problem. a) 0509 WWW UUT‘EK M W
~—> b) 2.009 {I c) 1.719 I2~7R2 +2 =0
d) 0.299 ...
e) 1.439 7"? R2 "‘ 2'0‘2‘ 16. Two light bulbs, one of resistance 20 Q and the other of resistance 60 Q,
can be connected either in series or in parallel to an emf of 110 V. Which
bulb is brighter in each connection? ——9 a) Series: 60 Q is brighter; Parallel: 20 Q is brighter
) Series: 20 Q is brighter; Parallel: 20 Q is brighter
c) Series: 20 Q is brighter; Parallel: 60 Q is brighter
d) Series: 60 Q is brighter; Parallel: 60 Q is brighter
e) The bulbs have the same brightness in each connection W: I M W) M W F": 12R'
:2.
Fanaééz' 1: VM W1 MW P’s L ...
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