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CU-Boulder PHYS 1120 - Sample Exam 2a solutions

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Exam II, vers. 0002 - Physics 1120 - Fall, 2007NOTE: THIS VERSION ONLY SHOWS THE ANSWERS. IF YOU WANT TO SEE THE SOLUTIONS, PLEASE DOWNLOAD V0001. THE ORDER IS DIFFERENT BUT ALL THE QUESTIONS ARE THERETA’s Name (Circle one!): Edwin Widjonarko, Craig Hogle, Mike Mullan, Mike Hermele, Naresh SenStarting time of your Tues recitation (write time in box: 9am, noon, 1pm, etc)Please do not open the exam until you are told to.Your exam should have 12 pages, numbered 1 thru 12.This exam consists of 18 multiple-choice questions worth 75 points total, and two multipart long-answer questions worth 25 points total. Fill in the bubble sheet with a #2 pencil. PLEASE follow all directions carefully. Remember to circle your TA's name above.Print and bubble in your name on the bubble sheet.Print and bubble in your student Identification Number.Print and bubble in your Exam version, 0001 or 0002, in the upper left of your bubble sheet in the area marked 1234.Erase mistakes as thoroughly as possible. Ask for a fresh bubble sheet if you fear you cannot thoroughly erase mistakes.After you begin, be sure to neatly print your name and ID on the long answer question pages.At the end of the exam, check that you have filled in the first 18 questions on the bubble sheet, with only one bubble filled in for each question.I have read and followed the instructions above.Signature______________________________________________Possibly Useful Constants: 1eV (electron-Volt) = 1.6⋅10-19 J. Magnitude of the charge of the electron e = 1.6⋅10-19 C. 1Ver 0002In all problems on this exam, please assume V=0 at infinity.The questions 1, 2, and 3 refer to the figure below.1. The figure shows some equipotential lines in a region of space. A proton is released from rest at point A. In which direction will it initially start moving?A) rightB) upC) downD) The protom will remain at restE) The proton will drift along the 0 V contour line, in either direction2. At which of the three labeled points (A, B, and C) is the electric field the strongest?A) At the point AB) At the point BC) At the point CD) It is equally strong at points B and C (but is stronger at both of those than at point A)E) It is equally strong at points A and B (but is stronger at both of those than at point C)3. Now consider moving a proton from point B to point C with constant velocity. How much external work would this take? (NOTE: answers are in units of eV. Be careful to think about both sign and magnitude) A) -1.6 ⋅10-19 eV B) +6 eV C) -6 eV D) +3 eV E) -3 eV 4. A circuit consists of a battery and three resistors, as shown. The current through each of the resistors is I. What is the current through the battery?A) 0 B) IC) (1/3)ID) 3IE) None of these0 V1 V2 V4 V-1 V-2 V-3 V-4 VC3 VABIII2Ver 00025. A wire carries a current of 16A.How many electrons pass through this wire in 1 minute? (Please note: that’s 1 minute, not 1 second)A) 16B) 1.5⋅10-16C) 1.0⋅1020D) 1.6⋅10-19E) 6.0⋅10216. Four charges are located at the corners of this square, as shown. What is the electric potential at the very center of the square? (d=r/2)A) B) C) D) E) 7. A small surface (S1) is completely enclosed by a larger surface (S2).The volume inside the first surface is V1. The volume in between the first and the second surface is V2. The total outward electric flux through the smaller surface S1 is positive, but the total outward electric flux through the outer surface S2 is negative. What is the sign of the total charge contained within the volume V2?A) PositiveB) ZeroC) NegativeD) Impossible to tellV1V2S1S23Ver 0002V = −2kQ√2dV = −6kQ√2dV = −kQ√2dV = 0V = kQ√2d−2Qrr+Qr+Q−2Qr8. An electron is injected between the plates of a large parallel plate capacitor, exactly in the middle, with initial kinetic energy KE(i) just as it enters the capacitor. Assume the magnitude of the electric field between the plates has a uniform value E between the plates. Which expression below gives the final kinetic energy KE(f) of the electron, just before it hits the lower plate?A) KE(f) = KE(i) + eEd/2 B) KE(f) = KE(i) - eEd /2 C) KE(f) = KE(i) + eEd D) KE(f) = KE(i) - eEdE) KE(f) = KE(i)9. You have two resistors, each made of identical materials. The second one, R2, is twice as long, but only half as thick as the other (that means its diameter is half as big). What is the ratio of their resistances? R1 R2A) R2 / R1 = 8B) R2 / R1 = 4C) R2 / R1 = 2D) R2 / R1 = 1E) R2 / R1 = 1/2−e+ + + + + + + + + + + - - - - - - - - - - - - - - - - d+Q−Q4Ver 000210. The figure on the right shows Electric field lines in a region of space. Which one of the figures below best shows the equipotential contour lines in this same region of space? A) B)C) D)E) E5Ver 000211. An ideal parallel plate capacitor has charge Q on its top plate, and -Q on the bottom plate. A thick neutral metal slab sits midway between the plates, as shown. The capacitor plates are electrically isolated, as is the metal slab. The metal slab has no net charge.How would the magnitude of the electric field at point A (located in the empty space between the lower capacitor plate and the metal slab) change if the slab were removed? (Ignore edge effects) A) Increase B) Remain the same C) Decrease D) Impossible to tell12. How would the stored energy of the capacitor change if the slab were removed?A) Increase B) Remain the same C) Decrease D) Impossible to tell13.Two uniformly charged rods are each bent into a quarter circle of radius R. The left rod is positive, with total charge +3Q (smeared out uniformly along the rod). The right rod is negative, with total charge -Q (again smeared out uniformly)What is the potential at the point A, located at the center of the circle formed by the two rods?A) V= π k Q/R B) V = k Q/RC) V = 2 k Q/R D) V = 0E) V = 2 π k Q/R A R +3Q -Q 6Ver 0002d+Q-QAL14. Two charges, +Q and +2Q, are arranged as shown. What is the potential at the point A?+2QA2rr+QA) 2kQ/rB) 3kQ/(2r)C) 3kQ/rD) kQ/rE) 015.The circuit shown on the picture includes three identical capacitors, each having the same capacitance C. What is the effective total capacitance of this circuit?A) C


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CU-Boulder PHYS 1120 - Sample Exam 2a solutions

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