Practice Exam 2_A solutions

Practice Exam 2_A solutions - Exam 2 Foam j; '...

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Unformatted text preview: Exam 2 Foam j; ' ":74 1. A parallel-plate capacitor is charged with a battery that is then disconnected. If the plate separation of this charged capacitor is doubled: 7 A) the surface charge density on each plate is doubled g B) the electric field is doubled E - g /e C) the potential difference is halved — D) the capacitance is doubled E) @1173 offhe above ) Dwain 6,16% ‘0 2. A battery has an emf of 9V and an internal resistance of 29. If the terminal potential difference is greater than 9V: A) it must be connected across a small external resistance 5"gg'g“) {- ...— I} B) the current must be zero t d ’3) C) it must be connected across a large external resistance \ i D) the currentmust be out of the positive terminal _. "bu-ca...“ Ef'fhe current must be 01f t“of'tfie"'negative1erfiii'_hm?<=__flj 3. The current density is the same in two wires. Wire A has twice the free electron concentration (density) of wire B. The drift speed of electrons in A is: A) the e as that_g_f electrons in B ' _, ’9' B)@alf that of electrons in E ) J a; U0. '3' C) twice at o 6 cc on m D) four times that of electrons in B E) one-fourth that of electrons in B 4. A hexagonal loop of wire carrying a counter clockwise current of 2.0 A has 6 sides that are all 0.5 m long. A 0.7 T uniform magnetic field is parallel to the base. The net magnetic force on the 5 dashed sides has a magnitude of: A) 0.3ON / f 3* 2) B) 0.51 N NE; _ a. C) 0.41 N p F” D N Fa? Egaifii’ff‘ O ,x G) 0 f“ - f as ...-.---*"' . wu- _‘_ fi .1‘ ‘t d Page 1 at?» 7/7» 5. A triangular solid in the shape of a 10 m wide inclined plane has height 3 In, length 4m and ramp length 5 In. A uniform, horizontal magnetic field B = 0.05 T is parallel to the 4m length. What is the magnetic flux through the ramp face, shaded in the figure? (use standard convention for normal vector direction of a closed surface) 6. A parallel-plate capacitor, with air dielectric, is charged by a battery, after which the battery is disconnected. A slab of glass dielectric is then slowly inserted between the plates. As it is being inserted: A) the glass makes the plates repel each other Wflassnut r < C) a force attracts the glass into the ca acitor D) no force acts on the glass E) a net charge appears on the glass (9?. “a” 37:; “x 71665,” fig 94>C5 Page 2 , ' - s it 7. An electron and a proton each travel with e/gual speeds aiound circular orbits in the same uniform magnetic field, as shown in diagram (not to, scale). The field is into the page in the diagram. Because the electron is less massive than the proton and because the electron is negatively charged and the proton is positively charged, which statement is TRUE: (B into page) i e’ F £0 13 O .3» P A) the electron travels counterclockwise around the smaller circle and the proton travels clockwise around the larger circle B) the electron travels clockwise around the larger circle and the proton travels counterclockwise around the smaller circle C) the electron travels counterclockwise around the larger circle and the proton travels clockwise around the smaller circle D) the electron travels ecunterclockwise around the smaller circle and the proton travels counterclockwise around the larger circle _ _ _ _ . 1 E) the electron"ti‘avels "clockwise aronnd't‘he smaller circle and the proton travels counterclockwise around the larger circle. J . . __,_,_....._....‘—...‘-—.u...._.-......__‘_.......-..-——-.—‘ __._.. - - run—qua...“ ..........__——-—-- _..__-._._......-_..—- 8. Which of the following graphs best represents the current-voltage relationship of a typical ?5 Watt incandescent light bulb in a reading lamp? . ' . xx“ ‘ I I I I I . ‘ It . F I |: '4 Dist-wow)“ .. :ngw V j; F V 31:? {1,333 C . _ 3.125300% -- Bewith win-wages «J 101M ° I II I” Ly b V A) IV [T V 3 IV \ may. {WWM *TDTWLL?’ D) v - E) II Page 3 12. The diagram shows a straight wire carrying a flow of electrons into the page. The wire is between the poles of a permanent magnet. The direction of the magnetic force exerted on the wire, and any electric potential difference between the top of the wire and the bottom of the wire are; Jim .. N O S f l I Fem OW“ A m < Vbot 3': C) ! vtop = ' bot a? < D) L , Vmp > Vbot E) T ’ Vtop > Vbot l3. Capacitor C1 is connected separately to a battery and charged until the magnitude of the charge on each plate is qo. Then it is removed from the battery and connected to two uncharged capacitors C2 and C3 as shown. The magnitude of the final charges on the capacitors are related by: 90=Q2+Q3,Q1=92 D) qu = Q) + Q3, q1 = 0' ' x‘a‘xx E) 90 = 971+ 632 + 633 14. A square loop of wire carrying 0.5A has a side length of 0.1m. The magetic dipole moment initially makes an angle of 90° with a 0.5-T magnetic field. As it turns to 0 ' ned with the field, the work done by the magnetic field is: ( A 2.5 x 10‘ J I ~ -- _ — V .134.ch E “d: and 9M B) 2. x J _ —3 , _ s a... I. Myjpstmmrm E) 1.0x10‘ J . 2.11.: with!» 103 raw/@ng) 0 -3 PageS : C. T 57% 425% 872. @4570 15. A certain ammeter has an internal resistance of 1 Q and a range from 0 to 50 mA (1 m = 10'3A). To change its range to 0 to 5 A, i.e. make its full scale reading be 5.0 A, we need to add: 5/? f g a i A) a resistance of 99 Q in parallel .__) . 05/} B) a series resistance of 99 D D) anextr I. - lare 53 169) series resistance‘aLS'm-‘DSJA @4401 L E $ 3 I ) a re31stance of U99 (2 n1 arall __ . 05 _ Sl— C) a resistance of 111000 9 in parallel 16. An initially uncharged capacitor C is connected in series with a resistor R and a battery of emf V0. Sufficient time elapses so that a steady state is reached. Which of the folio ' is TRUE? " i 51!! '9? AWme mm I g? 3 B) The time constant depends on V0 W W 6, W C) The total energy dissipated by R is independent of V0 (fare D) The initial current (just after the battery was connected) depends on C _ C 91d, Oil/Vila I l E) The final charge on C depends on R Mix/«WM a 17. A proton is in a region where a uniform electric field of 5 x 104 Vim is perpendicular to a uniform magnetic field of 0.8 T. If its acceleration is zero then its speed must be: % wwjm m . C)1.x s anvueu —- E13 / sow we- as» 0% )3 18. Two Ohmic resistors R1 and R2 dissipate 25 W and 100 W respectively when connected in parallel to a 100 V EMF. If instead we connect them in series to the same EMF then the power dissipation rates are for R1 and R2: - soon A) SOWabd 12.5w 7’— _. B) 12.5Wand50W J99: 25W R" R: C 4Wand16W 2 — was re..- m. ‘ semis I?» i M Mai? :— w) Ezra—mu l9. Capacitors with capacitances of C and 2C are initially uncharged and connected to each other through a resistor,R and open switch. The smaller capacitor is separately charged with Q and the switch is then closed. What is the time constant for the charge redistributionlsharing and how much energy has been dissipated as heat in the resistor once the potential difference across the capacitors becomes the same? meggfle El “35—- ‘24” l 1"” ya RC A) (213mg '(112) 21C . B) 3Rc, (wag/C C) 2RC, (4!9)Q2§C CezDI—fl’sc- D) R 1! I ‘2. ' a! 1-... — l 7“ a m at g- 3% Hf... 20. In a certain mass spectrograph, an ion beam passes through a velocity filter consisting of mutually perpendiCular fields E and B. The beam then enters a region of another magnetic field B‘ perpendicular to the beam. The radius of Curvature of the resulting ion roportional to: C) 5333' 117 ....I $01 ’ - rs ) WM 53 3:12? 3 5 Page 7 9. A Specially prepared semiconductor has a free electron density of 1 x 1027 electronsfnf’ and free holes (a mobile entity with charge = +e) with density of 5 x 1027 holesfm3. In an electtic field of l x 104 Wm the electrons have drift speed of 3 x 10"4 Infs and the + ‘7 holes have drift speed of l x 10“4 mils. The resistivity of this material (in (2-111) is 7‘1‘ 1’ about: . . .. E -. "Emwflam A) 0.05 ’ :. + I -' ‘7'- -- . I. B) 0.0. J” J‘ J""’" S J 6(ntgo‘rflafin) C) -08 H D) 0.11 :3. J0 E) 0.03 (Haggai [6&131’0 +‘15rJii7a‘lD J -:::§ 0.079.0_-m 10. A portion of a circuit is shown, with the values of the currents given for some branches. What is the direction and value of the cun‘ent i? A) l, 4A B) l, 2A C) T, 4A D) T 6A B) l,6A 11. A certain capacitor has a capacitance of 6 pF. After it is charged to 5 ,uC and isolated (the charging battery removed), the plates are then pulled apart so its capacitance becomes 2 11F. The work done by the pulling agent is about: 0 43 / A) 4x10 J fl v-I, __ x1 I " I D) - 2.. ,l a E)12x10$l I»! L ,.... 5 am a. L“ _ 2... r” . 3.: l/ _- 1 2p}; (3)1}? ...
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This test prep was uploaded on 04/04/2008 for the course PHYSICS 240 taught by Professor Davewinn during the Spring '08 term at University of Michigan.

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Practice Exam 2_A solutions - Exam 2 Foam j; '...

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