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Physics 112 Exam 2 Summer 2008 Name_________________ Physics 112 Summer 2008 Exam 2 Monday, July 14 Directions: This 60-minute exam consists of twenty multiple-choice questions. This test is worth 20% of your final grade. (One point is equal to 1% of the final grade.) The questions on this test are not in order of difficulty. You must mark all of your answers on both your test and answer sheet. In marking the multiple choice bubble sheet use a number 2 pencil. Do not use ink. If you did not bring a pencil, ask for one. Fill in the appropriate circles completely. If you need to change any entry, you must completely erase your previous entry. Carefully read each question and its five possible answers. Select one and only one answer for each question. Choose the answer that is closest to the correct one. You have a sheet in the back of the test that you may use for any extra rough work. When you are finished with the exam, place all exam materials, including the bubble sheet, the exam itself, and scratch paper that you used for the exam, in your folder and return the folder. Good luck! Page 1 of 9 Physics 112 Exam 2 Summer 2008 1. A charged particle moves with a constant speed through a region where a uniform magnetic field is present. If the magnetic field points straight upward, the magnetic force acting on this particle will be maximum when the particle moves A) straight upward B) straight downward C) upward at an angle of 45° above the horizontal D) downward at an angle of 45° above the horizontal E) in a plane parallel to the Earth's surface Answer: E 2. A proton has a speed of 3.0 × 106 m/s in a direction perpendicular to a uniform magnetic field, and the proton moves in a circle of radius 0.20 m. What is the magnitude of the magnetic field? A) 0.16 T B) 0.24 T C) 0.32 T D) 0.64 T E) 0.96 T Answer: A 3. A beam of electrons is accelerated through a potential difference of 10 kV before entering a velocity selector. If the B-field of the velocity selector has a value of 0.010 T, what value of the E-field is required if the particles are to be undeflected? A) 2.3 × 103 V/m B) 2.3 × 104 V/m C) 2.3 × 105 V/m D) 5.9 × 105 V/m E) 7.2 × 106 V/m Answer: D 4. A wire carries a current of 10 A in a direction of 30° with respect to the direction of a 0.30-T magnetic field. Find the magnitude of the magnetic force on a 0.50-m length of the wire. A) 0.75 N B) 1.5 N C) 3.0 N D) 6.0 N E) Zero Answer: A Page 2 of 9 Physics 112 Exam 2 Summer 2008 5. Consider two current-carrying circular loops. Both are made from one strand of wire and both carry the same current, but one has twice the radius of the other. Compared to the magnetic moment of the smaller loop, the magnetic moment of the larger loop is A) 16 times stronger B) 8 times stronger C) 4 times stronger D) 2 times stronger E) The same Answer: C 6. A circular loop of wire of radius 0.50 m is in a uniform magnetic field of 0.30 T. The current in the loop is 2.0 A. What is the magnetic torque when the plane of the loop is parallel to the magnetic field? A) zero B) 0.21 m∙N C) 0.41 m∙N D) 0.47 m∙N E) 0.52 m∙N Answer: D 7. Two long parallel wires are 4.0 cm apart. Each wire carries the current 10 A in the same direction. Find the magnetic field halfway between the wires. A) 10-4 T B) 2*10-4 T C) 3*10-4 T D) 2*10-2 T E) zero Answer: E 8. Two long parallel wires carry equal currents. The magnitude of the force between the wires is F. The current in each wire is now doubled. What is the magnitude of the new force between the two wires? A) 4F B) 2F C) F/4 D) F/2 E) F Answer: A Page 3 of 9 Physics 112 Exam 2 Summer 2008 9. How many turns should a 10-cm long solenoid have if it is to generate a 1.5 × 10-3 T magnetic field on 1.0 A of current? A) 12 B) 15 C) 119 D) 1194 E) 3000 Answer: C 10. Doubling the number of loops of wire in a coil produces what kind of change on the induced emf, assuming all other factors remain constant? A) The induced emf is 4 times as much. B) The induced emf is twice times as much. C) The induced emf is half as much. D) The induced emf is one qorter as much. E) There is no change in the induced emf. Answer: B 11. As a coil is removed from a magnetic field an emf is induced in the coil causing a current to flow within the coil. This current interacts with the magnetic field producing a force which A) acts at right angles to the coil's motion. B) acts in the direction of the coil's motion. C) causes the coil to tend to flip over. D) acts in the direction opposite to the coil's motion. E) the force is zero Answer: D 12. A circular coil lies flat on a horizontal table. A bar magnet is held above its center with its north pole pointing down. The stationary magnet induces (when viewed from above) A) no current in the coil. B) a clockwise current in the coil. C) a counterclockwise current in the coil. D) a current whose direction cannot be determined from the information given. E) alternating current Answer: A Page 4 of 9 Physics 112 Exam 2 Summer 2008 13. A bar magnet falls through a loop of wire with the north pole entering first. As the north pole enters the wire, the induced current will be (as viewed from above) A) zero B) clockwise C) counterclockwise D) to top of loop E) ) a current whose direction cannot be determined from the information given Answer: C 14. The coil of a generator has 50 loops and a cross-sectional area of 0.25 m2. What is the maximum emf generated by this generator if it is spinning with an angular velocity of 4.0 rad/s in a 2.0 T magnetic field? A) 50 V B) 100 V C) 200 V D) 400 V E) 500 V Answer: B 15. A 4.0-mH coil carries a current of 5.0 A. How much energy is stored in the coil's magnetic field? A) 2.0 mJ B) 10 mJ C) 20 mJ D) 50 mJ E) 80 mJ Answer: D 16. The inductive reactance in an ac circuit changes by what factor when the frequency is tripled? A) 1/3 B) 1/9 C) 3 D) 9 E) 81 Answer: C Page 5 of 9 Physics 112 Exam 2 Summer 2008 17. What current flows in a 60-mH inductor when 120 V AC at a frequency of 20 kHz is applied to it? A) 8.0 mA B) 16 mA C) 24 mA D) 32 mA E) 64 mA Answer: B 18. An electric field is produced by a A) constant magnetic field B) changing magnetic field C) either a constant or a changing magnetic field D) changing gravitational field D) none of the given answers Answer: B 19. If the electric field in an EM wave has a peak value of 2.0 V/m, what is the peak value of the magnetic field? A) 6.7 nT B) 2.0 T C) 20 T D) 3.0 × 106 T E) 3.0 × 108 T Answer: A 20. Unpolarized light passes through five successive ideal polarizers, each of whose axis makes a 45° angle with the previous one. What fraction of the light intensity is transmitted? A) 1/2 B) 1/4 C) 1/8 D) 1/16 E) 1/32 Answer: E Page 6 of 9 Physics 112 Exam 2 Summer 2008 Scratch Paper (intentionally left blank) Page 7 of 9 Physics 112 Exam 2 Summer 2008 Record Sheet You may fill in this sheet with your choices, detach it and take it with you after the exam for comparison with the posted answers 1 E) in a plane parallel to the Earth's surface 11 D) acts in the direction opposite to the coil's motion. 2 A) 0.16 T 12 A) no current in the coil 3 D) 5.9 × 105 V/m 13 C) counterclockwise 4 A) 0.75 N 14 B) 100 V 5 C) 4 times stronger 15 D) 50 mJ 6 D) 0.47 m∙N 16 C) 3 7 E) zero 17 B) 16 mA 8 A) 4F 18 B) changing magnetic field 9 C) 119 19 A) 6.7 nT 10 B) The induced emf is twice times as much. 20 E) 1/32 Page 8 of 9 Physics 112 Exam 2 Summer 2008 Page 9 of 9