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Unformatted text preview: Physics 240 Fall 2007: Exam #3 Please print your name: 0 lac—« l Instructions l. Fill in your name above 2. This will be a 1.5 hour, closed book exam. The exam includes 20 questions. 3. You may use a calculator, please do not share calculators 4. You may use three 3”X5” note cards With notes and equations you think may be useful. You can write on both sides of the card if you like. Table of constants: 80 2 8.85 x 10‘12 (:2/Nm2
k = 1/(47t20) qelectronz16 X 1019 C
qproton216 X 1019 C
melgctmn:9.1 x 10‘“ kg
mpmmn=r67 x 10'27 kg
in = 471 x10“7 Tm/A G = 6.67 x 10‘“ NmZ/kgz Questions marked (P) have at least one answer choice that gives partial credit Version 3 Page 1 l. W A) P/9 U) WW% The primary of a 3:1 step—up transformer is connected to a source and the secondary is
connected to a resistor R. The power dissipated by R in this situation is P. If R is
connected directly to the source it will dissipate a power of: B) P/3
C) P D) 3P Vg
E) 9P Steiner. r'lmiﬁa 9% Skepwwg . A long, straight wire is in. the same plane as a wooden, nonconducting loop. The wire carries an increasing current I in the direction shown in the figure. In the wooden loop: . w a s!
A) There will be a clockwise induced emf, but no indulzed current. B) There will be a clockwise induced current in the loop.
C) There will be a counterclockwise induced current in the loop.
D) There will be no induced emf and no induced current.
E) There will be a counterclockwise induced emf, but no induced current. . An inductor and a 100 Q resistor are connected in series to a 10 volt battery through a switch. At the instant after the switch is closed it is noted that the current in the circuit
is changing at a rate of 50 amps/second. What is the inductance of the inductor? A) 0.5 H
B) 0.07 H
C) 2.2 H
D) 0.l H
E) 0.2 H Version 3 Page2 4. The diagrams show three circuits with identical batteries, identical inductors, and
identical resistors. Rank them according to the current through the battery just after the
switch is closed, from least to greatest. ‘8 1
A) 2and3ties,then1
W9 B) 1,3,2
C) 1,2,3
D) 3,1,2
E) 3, 2,1 Version 3 Page 3 5. A copper penny slides on a horizontal frictionless table. There is a square region of
constant uniform magnetic ﬁeld perpendicular to the table, as shown. Which graph
correctly shows the speed v of the penny as a function of time t? in :3 vi aw ' “a 1’ “any . 3 2‘ 3 A)1 {Amalia Mm is New?” WW gunman“;
VJ! it.
ii iii {'3 Lamina CM a. MM i' 151 l, TW': 51%. ﬁne??? MM“ iéyﬁg WNW gamma é’e w lemma W ﬁwﬁiﬁli “3% 6. A parallel—plate capacitor has closely spaced circular plates of radius R = 2.00 cm.
Charge is ﬂowing onto the positive plate at the rate I = dQ/dt = 1.36 A. The magnetic
field at a distance r = 2.00 cm from the axis of the plates is approximately
A) 25.6 uT ' _ Q
B) 8.83uT Wk is m wamﬁk A» gigpiaaaaw
L 45.7HF M$ 5“ g Wyﬁ E) 1.65 trr M lgaei’amﬁe. : 7. Immediately after switch S in the circuit shown is closed, the current through the battery
shown is: A) o
B) Vo/R1
C) Vo/R2
Ms D) VO/(R1+R2)
E) V0(R1+R2)/(R1R2) Use the following to answer question 8: A power plant produces a peak voltage of 6.0 kV and a peak currentl 50 A. The voltage is
stepped up to 240 kV by a transformer before it is transmitted to a substation. The resistance of
the transmission line betwe en the power plant and the substation is 75 Q. 8. What percentage of the power produced at the power plant is lost in transmission to the
substation? A) 0.34 % B) 0.23 %
eme§ C) 0.12 %
D) 0.47 0/0 E) 0.41 0/0 WW a as aw
Version Page 5 9. A metal ring is dropped from rest below a bar magnet that is fixed in position as
suggested in the figure. An observer views the ring from below. Which one of the
following statements concerning this situation is true? smasher};
bar magnet A) Since the magnet is stationary, there will be no induced current the ring. B) As the ring falls, there will be an induced magnetic field around ti ring that
appears clockwise as viewed by the observer. C) As the ring falls, an induced current will flow clockwise as viewed by the observer. ’W’ﬁ‘rD) As the ring falls, an induced current will flow counterclcckwise as viewed by the
observer. E) As the ring falls, there will be an induced magnetic ﬁeld around the ring that appears caunlcrclocsz’sc as viewed by the observer. Version 3 Page6 10. R x 2 LS 14$? X 3?“:
At a certain instant in time, a wire rod rolls with a speed of 20 m/s on two metallic rails, 1.0 m apart, that form a closed loop. If the uniform magnetic field is 1.5 T into the
page, then what is the rate at which the resistor dissipates energy?
A) 33 mW
WmﬁB 76 mW .
) a m W W Q L, C) 50mw W «w
a . . a we
iii 23% a?" a... Wﬁﬁﬁt} @QWW‘; {i}
Pa was» m /@ “ WK “t” g £‘lrggg m 7% MW l 1. In a sinusoidally driven series RLC circuit, the inductive reactance is XL = 200 Q, the capacitive reactance is XC = —100 Q, and the resistance is R = 50 Q. The current and
applied emf would be in phase if: A) the resistance is increased to 200 Q, with no other changes IX» 1;,” 3:: W L m, mamﬂ‘ B) the inductance is reduced to zero, with no other changes W m g
WC) the capacitance is halved, with no other changes %g ‘7’” m” m 5% a ﬁx“
D) the resistance is increased to 100 Q, with no other changes my”: % a in g a w 7 Ah _
E) the capacitance is doubled, with no other changes ‘@ m l W §m$ a. “X3” “’3” “Mama
gﬁ L» ki @W" g gwmég gggw Mﬁaémg newter 12. The plane ofa wire loop with an area of0.20 m2 is perpendicular to a magnetic ﬁeld of
50 mT. in 40 ms you rotate the loop 90°80 that its plane is parallel to the magnetic
field. The average emf induced in the loop is A) 4.0V
was) 0.25v
C) 6.0V N mats
D) 1.3V W
5) 3.0V
at M...
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Er M3 W W atméw'ﬁ Version 3 Page 7 Use the following to answer question l3: A 100—111 long solenoid has a radius of 0.050 m and 1.50 x 105 turns. The current in the solenoid
changes at a rate of 6.0 A/s. A conducting loop of radius 0.0200 n1 is placed at the center of the solenoid with its axis the same as that of the solenoid as shown.
j i salenoid fr“r“‘"‘rjrt~in":r"‘;~*”‘i":‘"‘i"tfi‘WW3“?
ifllili {‘1 if i if ii
u W y "i
.iil irrigiilrtmilliiit l1
(‘1, 2".)vy3&1‘5.13\n}séa,ejgxJVLisaﬁJAﬁwisfxxx/k, vii ‘ AVA” J aﬁj‘vj‘uﬁw‘g’hj} 2?
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l3. Determine the mutual inductance of this combination. Qua 4, kw“ ! W V 5
M X 104 H m WWWWWWMMW : B) 4.4 x 10‘4H x E
C) 5.9 x 10"4 H A, g m
. a r M
D>36><10‘4H M as: Ema: ,,,, ML“ P .. ,AAmE Wm“
E) 1.8 x 10‘4 H “t i an 14. An ac generator produces 10 V (peak) at 400 rad/s. It is connected to a seriesRL circuit
(R = 17.3 Q, L = 0.025 H). The peak current is: I A) 0.50 A and leads the voltage by 30° if“ m‘ \ as
B) 0.71 A and lags the voltage by 30° v: We ( gﬁhgfh } “WI?”
C) 1.40 A and lags the voltage by 600 at?!
AvaQ D) 0.50 A and lags the voltage by 30° ' g: i gagfjgy gm. “W g. 32“)?
E 0.5 A th 1 b ° W '
) ‘8 and leads e vo tage y 90 if”! ( gm «:3 Mi
w; i a“?
larli a *QmL‘E e gﬁaﬁw \ M”
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Lymam" mm is... magma
t %«3 egg}... @134» 41» ée Wﬁwouék .: $.34“, ~I(%F)x :4 >£3aa§ Version 3 Page 8 15. When the switch S in the circuit shown is closed, the time constant for the growth of
current in R2 is: . . l A gets Shﬁrfuﬁ?
we.» :3) m U'l huge, eacMﬁSS Em é‘m‘“ Kw" y a?
C) L/(R1+R2) r’s .ﬁ {mat 5%, M rmygmmﬁ é, imgwbwniv
D) L(R1+R2)/(R1R2) Him a g .
E) (L/R1+L/R2)/2 ” I; , 3., m MW ngvmol— % s {Weigh l6. (P) A hollow cylindrical conductor (inner radius = a, outer radius = 29) carries a currenti
uniformly spread over its cross—section. Which graph below correctly gives B as a
function of the distance r from the center of the cylinder? 33’! 3; Bi l
i r "‘ r ] 2 V 1
a £2 ~~ , C
l r Q .3» 5'
3‘ A) i
B) it wwa) in
D) IV
E) v
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Version3 Page9 $5.4” x: g, i g E? m a? e“ gm 9 m a. ﬁg 3;? W 3 . givegacﬁ m g9 geﬁ Q g égﬁ’lw “We”; heirwﬁﬁh ,, ggéﬂtﬁé. £23
mm EM MA; Lg & ﬂﬂ;i¢m:’mm 17. A cylindrical region of radius R contains a uniform magnetic field, parallel to its axis,
with magnitude that is changing linearly with time. If r is the radial distance from the
cylinder axis, the magnitude of the induced electric ﬁeld inside the cylinde r (r<R) is
proportional to:
A)R ggdvﬁw “Jig
C) r2
D) l/r2 we 3
. ﬂ 1'.
E) 1/, 93> E‘seﬂwwﬁwp akaf»
a: m w {3
18. The primary coil of a step down transformer with a 10:1 turns ratio is connected to an
AC generator with a peak voltage of 120V. If a $2 resistor is connected in series with
the secondary coil then how large will the peak current through the generator be?
A) 0.5 Amps
B) 1.7 Amps
“~9C) 0.24Amps E a", g.“ . _ . 5'
D) 17Amps ir‘wg “"5 ’1‘, SM ﬁﬁlmdmgg
E) 24 Amps GM: W‘} {5 vi» M42, Pam”: Version 3 Page 10 19. An LC circuit has a capacitance of 30 uF and an inductance of 15 mH. AT time 1‘ = O the
charge on the capacitor is 10 HC and the current is 20 mA. The maximum charge on the capacitor is: gﬁgmw 4w “gage A) 8.9 uC
B) 10 MC at g” "t" at. éa
2:: i “it”
C) 12 MC wafﬂes f a”
W”??? D) 17 nC “8%? r
E w .33 Q i “W
) 24m: glam {ML £9
20. Four closed surfaces are shown. The areas Amp and AM of the top and bottom faces and
the magnitudes Em and Bbot of the uniform magnetic fields through the top and bottom
faces are given. The fields are perpendicular to the faces and are either inward or
outward. Rank the surfaces according to the magnitude of the magnetic ﬂux through the
curved sides, least to greatest.
31mm ’2‘ em?“
1?,” x 2’ ml: inward
2 _
‘énﬁi “x 2 mg g m 9%» W... 5’ == 1»: mt; gamma 5 a? mi“. 1; mama ‘ "’
{gagg mgmngaw kw: am 4 i w
am: am”: aw 2 emf
8m: 3' mTJriwanj ifng 3 me inward W
I" ‘1‘
am: 2 cm *
n e :2: m? amwanj ...
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 Fall '08
 DaveWinn

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