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54S07Exam1Solution

# 54S07Exam1Solution - Physics 54 SW Exam l Name j W Section...

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Unformatted text preview: Physics 54- SW Exam l Name j W Section: Part1: Short Answer :3? (3pm each) Circle the correct answer: 1) I [email protected] A conical volume contains a charge q = 8.85 14C. The ﬂux out of the circular base is. 3 X 105 N—mz/C. The ﬂux out of the rest (top) of the cone is: a) O. b) -7 x 105 N-mz/C c) — 3 x 105 N—mZ/c + 7 x 105 N—mz/C Use 80 = 8.85 x 10'”. 2) A hollow conducting sphere with a net charge of zero has a charge q inside. The charge on the outer surface of the sphere is: a) ‘Q- gawk" g???) gangmcM 6”? /ﬁ@@“’ \$w¢§§f€ 3) 5 10 Two graphite resistors with identical resistivity are shown. Which has the larger resistance? (numbers denote sizes in mm) 6,. sag a) The left one. jg a ﬂag b) The right one. ﬂ; .5: we??? (9 Cannot determine. 6” ﬂ? oth have the same resistance. g «g: “57%?” a” j at em 4) Two identical capacitors are separately charged to different voltages and then disconnected from the voltage sources. Then their + terminals are connected together and their — terminals are connected together. After they are connected: he sum of the positive charges on the + plates is conserved. I The voltage between the + and ~ terminals on each capacitor is unchanged. c) The total energy is conserved. d) The voltage between the + and _ terminals is the sum of the initial voltages. -’ (/2 ~,, 3* Efﬁgy/fr . {31+ The equivalent capacitance of the combination on the left is: 2/1 ewe/7f} m/’%% C 0) Cklﬁif‘ 23 2C. 1? (l3) 3C/2. /:JJ5’ Q? a" gag; c) 2C/3. " 20V 10V an i??? wteg 5%” We The sphere on the left is held at an electric potential of 20 V, while the plate on the right is held at 10 V. A particle with a charge of 1 HC moves from the surface of the sphere to the plate. In doing so: a, 1%; e; {agave/Eire é ‘ r1 gains 10 M of energy b) It loses 10 pl of energy c) It loses 20 ”J of energy. (1) It is repelled more by the plate than by the sphere. 7) A l————-—- +Q -Q A d An air—spaced capacitor with plates of area A and separation d is charged as shown and is left connected to the battery that was used to charge it. I move the plates closer together. a. 6, \$ é a The stored energy remains the same and the charge Q increases. W g E g V ’ The field increases, and the stored energy increases. g g V f c The charge Q cannot change, since the plates are conductors. a, d) The charge decreases and the stored energy increases. 5 ”g 8) Ten 1 W light bulbs are connected in parallel to a battery One of the light bulbs fails The total powe1 dissipated 1n the remaining bulbs 18: a) 0 because all of the bulbs go off. ﬁng ,1. - Cannot be determined. M 9 W because each of the remaining bulbs still receives the same voltage. 10 W because the current through each bulb increases to compensate. 9) The surface of a charged copper plate is at an electric potential of 10 V. The electric potential anywhere inside the plate: g =3 «a % a» g a: 235% £3 ”é y? l Is 10 V because the electric field is zero inside the plate. ) Is 0 inside the plate. 3;: Q (3) Cannot be determined from the information given. d) Depends on the distance from the surface of the plate. 10) Six identical positive charges are arranged symmetrically in the form of a hexagonal ring. The electric field exactly in the center: a) Depends on the size of each charge. b) Is positive and points outward. Is negative and points inward. Is exactly zero. Part ll: Long Answer l) (25 pts) Two positive 1 LLC charges sit on the x-axis, each one meter from the point P shown. An additional negative charge ~Q1 sits on the y-axis. Use sin30=0.500, cosso=0.866, k = 9 x 109 . a) For Q1 2 0, what are the components of the net field vector at the point P arising from the two positive charges alone? b) If a 1 [1C charge is put at the point P with Q; = 0, what is the net force vector on it arising from the two positive charges. 0) If negative charge on the y~axis is — 4 LLC, how far from the point P should it be placed so that the net force on any charge placed at P would be zero? a6 W} a} as: ME: 2: trim” a ail/935% Es Eyg 5': Q gy g E; mega agatiaa {if} &> gig /@¢6g if; ﬁat/Q” j“ M WV?” «39% @ @Wﬁwﬁg gm Q {g} at} E), g @949 as» ﬁaww ae— 2) (25 pts) Three conductors each with a large surface area A = l m2 are charged as shown, with Q = 0.885 uC. The conductors are spaced by d1 = 0.01 m and d2 = 0.02 m. The center conductor is held at zero potential. Use 80 = 8.85 x 1042. a) Show on the picture how the charges are distributed on the three conductors. b) Draw arrows showing the direction of the fields E1 and E2 and find the magnitude of each field in terms of the relevant given quantities. 0) Similarly, find magnitude and sign of the electric potentials V1 and V; at terminals 1 and 2. d) If terminals 1 and 2 are connected, does charge redistribute? Explain why or why not. Eﬁgfgﬁ’ a; ﬁg? 3) (ZOpts) An 8 volt battery is connected to a network of resistors as shown. Each resistor is 10 Q. a) Find the equivalent resistance between points a and b. 13) Find the current I. c) Find the voltage drop across the resistor connected to point b. ...
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54S07Exam1Solution - Physics 54 SW Exam l Name j W Section...

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