Physics 106 Sections 1 & 2 Midterm Exam #1 Fall 2011

Transcription

1 Physics 106 Sections 1 & 2 Midterm Exam #1 Fall 2011 Instructor: Lawrence Rees 3-digit CID: This test has 25 problems. The CID is only used to return your test. If you forgot it, make one up. Do NOT write your name. Mark the best answer for each problem on the answer form (the bubble sheet). Be careful to enter your answers correctly. Your bubble sheet answer is your final answer. No notes or written materials are allowed except for a foreign language dictionary. You may not use equations or other information stored in your calculator. Possible useful information = ଽ ܥ/ ଶ ଶ = ଵଶ ܥ ଶ /( ଶ) ܥ 10 ଵଽ 1.60 = Charging capacitors: = (1 ௧/( ) ) Discharging capacitors: = ௧/( ) The long side of a battery is positive. = 10 ଽ = 10 ଵଶ ܩ = 10 ܯ = 10 = 10 ଷ = 10 ଷ ߤ = 10 ଵଶ = 10 ଽ 1. Two charges are placed along the x axis of a coordinate system. A charge of +20 μc is placed at x = +20 cm and a charge of 20 μc is placed at x = 20 cm. What is the force on a test charge of +2 μc located at the origin? A N in the +x direction B N in the x diection C N in the +x direction D N in the x diection E N in the +x direction F N in the x diection G N in the +x direction H N in the x diection I. The force is zero. 2. Two charges are placed along the x axis of a coordinate system. A charge of +20 μc is placed at x = +20 cm and a charge of 20 μc is placed at x = 20 cm. What is the electric field at the origin? A V/m in the +x direction B V/m in the x diection C V/m in the +x direction D V/m in the x diection E V/m in the +x direction F V/m in the x diection G V/m in the +x direction H V/m in the x diection I. The electric field is zero.

2 3. Two charges are placed along the x axis of a coordinate system. A charge of +20 μc is placed at x = +20 cm and a charge of 20 μc is placed at x = 20 cm. What is the electric potential (voltage) at the origin? A ହ V in the +x direction B ହ V in the x direction C ହ V D V in the +x direction E V in the x direction F V G. The electric potential is zero. 4. An egg-shaped conductor is grounded. A large charge is brought near the conductor. Which of the following is true of the conductor after the system comes to electrostatic equilibrium? A. The total charge on the conductor must be zero. B. the electric field inside the conductor must be zero. C. Any charges on the conductor must be uniformly distributed. D. The sum of all forces between the conductor and the charge must be zero. 5. A Ca 2+ ion with charge +2e is released from rest and moves due to the force from an electric field. It moves from a position in the field having an electric potential of 14 V to a position having an electric potential of 8 V. The final kinetic energy of the ion (when it reaches the 8 V potential) is: A. 6 ev B. 8 ev C. 12 ev D. 14 ev E. 16 ev F. 28 ev 6. Two parallel plates of a capacitor are charged by connecting them to a battery. If the plates are first disconnected from the battery and then moved farther apart, which of the following will occur? A. charge will increase B. charge will decrease C. voltage will increase D. voltage will decrease

3 7. If R 1 < R 2 < R 3, and if these resistors are connected in parallel in a circuit, which resistor dissipates the most power? A. R 1 B. R 2 C. R 3 D. All dissipate the same amount of power. 8. Three resistors connected in parallel have the individual voltages of V 1, V 2, and V 3, respectively. Which of the following expresses the total voltage V T across the three resistors when connected in this manner? A. V T = V 1 + V 2 + V 3 B. V T = (1/V 1 + 1/V 2 + 1/V 3 ) C. V T = V 1 = V 2 = V 3 D. 1/V T = 1/V 1 + 1/V 2 + 1/V 3 9. If I 1 goes to the right through R 1, I 2 goes to the right through R 2, and I 3 goes to the right through R 3, what is the equation resulting from applying Kirchhoff's loop rule for a clockwise loop around the perimeter of the circuit? A. 8.0 V + I 1 R 1 + I 3 R 3 = 0 B. 8.0 V + I 1 R 1 I 3 R 3 = 0 C. 8.0 V + I 1 R 1 + I 3 R 3 = 0 D. 8.0 V I 1 R 1 + I 3 R 3 = Consider two long, straight parallel wires, each carrying a current I. If the currents are flowing in the same direction: A. the two wires will attract each other. B. the two wires will repel each other. C. the two wires will exert a torque on each other, but no net force. D. neither wire will exert neither a torque nor a force on the other. 11. A positively charged ion is moving in the same direction as a magnetic field. The path of the ion is: A. a straight line B. a circle C. a spiral (helix) D. the path could be any of the above.

4 12. Think of a coil of wire going around the edge of this sheet of paper. Current goes counterclockwise through the coil. If the coil is placed in an external magnetic field going from the bottom of the paper toward the top of the paper, the coil will rotate so that: A. the top of the coil (the segment at the top of the sheet of paper) rotates toward you and bottom of the coil rotates away from you. B. the bottom of the coil (the segment at the bottom of the sheet of paper) rotates toward you and top of the coil rotates away from you. C. the right side of the coil (the segment at the right of the sheet of paper) rotates toward you and left side of the coil rotates away from you. D. the left side of the coil (the segment at the right of the sheet of paper) rotates toward you and right side of the coil rotates away from you. 13. A 1.50 F capacitor (a big capacitor) is charged by connecting it to a 12.0 V battery. It is discharged by connecting it to a light bulb that has a resistance of 10.0 Ω. How long does it take for half of the charge to leave the capacitor? A. 7.5 s B s C s D s 14. An electron is moving upward in a region where the magnetic field points to the south. The force on the electron is in what direction? A. north B. south C. east D. west E. up F. down 15. If the electron of the previous problem has a velocity of 385 m/s and the magnetic field has a strength of T, what is the force on the electron? A ଶ B ଵ C ଵ D N

5 16. Find the equivalent capacitance between points a and b in the diagram to the right. The values of the individual capacitances are as follows: C1 = 100 μf C2 = 200 μf C3 = 400 μf A. 31 μf B. 92 μf C. 131 μf D. 209 μf E. 229 μf F μf G μf H μf 17. If each capacitor in Problem 16 were replaced by a resistor, what would be the total resistance between points a and b? The values of the individual resistances are as follows: C1 = 100 Ω C2 = 200 Ω C3 = 400 Ω A. 31 Ω B. 92 Ω C. 131 Ω D. 209 Ω E. 229 Ω F Ω G Ω H Ω 18. In the circuit illustrated to the right, the value of R 1 is 3.0 Ω. What is the current passing through R 1? A A B A C A D A E A

6 19. In the circuit of Problem 18, what is the power provided by the battery? A W B W C W D W E W F W 20. Find the voltage at point A if the value of R 1 is 2000 Ω. A. 19 V B. +19 V C. 20 V D. +20 V E. 25 V F. +25 V G. 60 V H. +60 V 21. You wish to make a capacitor with a capacitance of at 90 pf. You have at your disposal only two aluminum plates and some paper. The aluminum plates are squares of length 3.50 cm on each side. The paper has a dielectric constant of κ=3.0 and a thickness of 60 μm. You cut the paper to match the size of the plates. How many sheets of paper do you need? (Assume that any small amounts of air between the plates may be ignored. Round your answer to the nearest integer.) A. 1 B. 2 C. 3 D. 4 E. 5 F. 6

7 22. The breakdown voltage of a capacitor is the voltage at which there will be electrical sparks through the dielectric. With a paper dielectric, these sparks leave holes in the dielectric that ruin the capacitor. The breakdown voltage of the capacitor in Problem 21 is 2800 V. What is the largest possible charge you could put on your capacitor? A pc B pc C pc D μc E μc F μc 23. Which of the following is NOT true concerning electric field lines? A. The direction of the electric field is along the tangent to the electric field line passing through a point. B. Electric field lines from two positive charges cross each other in the region between the charges C. Electric field lines are closer together where the field is stronger D. If a +2μC charge and a 1 μc charge are near each other, half the field lines from the positive charge go to the negative charge and the other half go off to infinity (assuming there are no other charges that we need to consider). 24. A block of material has dimensions of 8 cm 4cm 4cm. (a) When wires are connected to the ends of the block (a), it has a resistance of 16.0 Ω. What is the resistance when wires are connected to opposite faces of the block (b)? (b) A. 2.0 Ω B. 4.0 Ω C. 8.0 Ω D Ω E Ω F Ω 25. A velocity selector is a device we described in class that employs both electric and magnetic fields to affect the motion of a charged particle. A positive ion enters the velocity selector at the left. An electric field of 2000 V/m points upward, as shown in the figure. A magnetic field of 0.25 T (not shown) points out of the page toward you. If the ion travels through the device undeflected, what is the speed of the ion? A mm/s B. 500 m/s C m/s D m/s

8 Answers 1. H 2. B 3. G 4. B 5. C 6. C 7. A 8. C 9. D 10. A 11. A 12. B 13. B 14. D 15. B 16. E 17. D 18. C 19. F 20. C 21. F 22. E 23. B 24. B 25. D