Name: Class: Date: AP Physics Spring 2012 Q6 Practice. Multiple Choice Identify the choice that best completes the statement or answers the question.

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1 ame: Class: Date: ID: A AP Physics Spring 2012 Q6 Practice Multiple Choice Identify the choice that best completes the statement or answers the question. 1. (2 points) A potential difference of 115 V across the primary of a step-down transformer provides a potential difference of 2.3 V across the secondary. What is the ratio of the number of turns of wire on the primary to the number of turns on the secondary? a. 1:50 b. 50:1 c. 7:1 d. 1:7 2. (2 points) According to Lenz s law, the magnetic field of an induced current in a conductor will a. enhance the applied field. b. heat the conductor. c. increase the potential difference. d. oppose a change in the applied magnetic field. 3. (2 points) A coil with a wire that is wound around a 2.0 m 2 hollow tube 35 times. A uniform magnetic field is applied perpendicular to the plane of the coil. If the field changes uniformly from 0.00 T to 0.55 T in 0.85 s, what is the induced emf in the coil? a. 45 V b. 33 V c. 33 V d. 45 V 4. (2 points) Consider two long, straight, parallel wires, each carrying a current I. If the currents move 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. d. neither wire will exert a force on the other. 5. (2 points) Which compass needle orientation in the figure above might correctly describe the magnet s field at that point? a. a b. b c. c d. d 6. (2 points) A current in a long, straight wire produces a magnetic field. These magnetic field lines a. radiate out from the wire to infinity. b. come in from infinity to the wire. c. form circles that pass through the wire. d. form circles that go around the wire. 7. (2 points) In most electric generators, either the armature or the magnetic field is, generating a(n). a. rotated; induced current b. turned off; temporary dipole c. interrupted; impulse change d. nonconducting; flux line 8. (2 points) A current-carrying conductor in and perpendicular to a magnetic field experiences a force that is a. perpendicular to the current. b. parallel to the current. c. inversely proportional to the potential difference. d. inversely proportional to the velocity. 9. (2 points) In a permanent magnet, a. domain alignment persists after the external magnetic field is removed. b. domain alignment becomes random after the external magnetic field is removed. c. domains are always randomly oriented. d. the magnetic fields of the domains cancel each other. 10. (2 points) In a magnetized substance, the domains a. are randomly oriented. b. cancel each other. c. line up mainly in one direction. d. can never be reoriented. 1

2 ame: ID: A 11. (2 points) The operation of an electric motor depends on a. the Doppler effect. b. the photoelectric effect. c. the magnetic force acting on a current-carrying wire in a magnetic field. d. induced commutators from the rotation of a wire in a magnetic field. 12. (2 points) The lines of the magnetic field around a current-carrying wire a. point away from the wire. b. point toward the wire. c. form concentric circles around the wire. d. are parallel with the wire. 13. (2 points) An electric current that changes directions at regular intervals is called a. directional current. b. alternating current. c. reversible current. d. fluctuating current. 14. (2 points) In soft magnetic materials such as iron, what happens when an external magnetic field is removed? a. The domain alignment persists. b. The orientation of domains fluctuates. c. The material becomes a hard magnetic material. d. The material returns to an unmagnetized state. 15. (2 points) Which of the following statements about electromagnetic radiation is true? a. It transfers energy to objects in the path of the electromagnetic waves. b. It can be converted to other energy forms. c. It transports the energy of electromagnetic waves. d. all of the above 16. (2 points) A current-carrying wire 0.50 m long is positioned perpendicular to a uniform magnetic field. If the current is 10.0 A and there is a resultant force of 3.0 on the wire due to the interaction of the current and field, what is the magnetic field strength? a T b. 15 T c T d T 17. (2 points) Consider two long, straight, parallel wires, each carrying a current I. If the currents move in opposite directions, 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. d. neither wire will exert a force on the other. 18. (2 points) All of the following are ways to induce an emf in a loop of wire except which one? a. Move the loop into or out of a magnetic field. b. Rotate the loop in a magnetic field. c. Move the loop parallel to a magnetic field. d. Insert the loop into a changing magnetic field. 19. (2 points) A stationary positive charge, Q, is located in a magnetic field, B, which is directed toward the right. What is the direction of the magnetic force on Q? a. toward the right b. up c. down d. There is no magnetic force. 20. (2 points) A bar magnet falls through a loop of wire with constant velocity, and the north pole enters the loop first. The induced current will be greatest when the magnet is located so that the loop is a. near either the north or the south pole. b. near the north pole only. c. near the middle of the magnet. d. With no acceleration, the induced current is zero. 21. (2 points) A microscopic magnetic region composed of a group of atoms whose magnetic fields are aligned in a common direction is called a(n). In most materials, when these groups are randomly distributed, the substance will show no magnetism. a. domain; no b. pole; some c. cell; unusual d. ion; strong 22. (2 points) Which conversion process is the basic function of the electric generator? a. mechanical energy to electrical energy b. electrical energy to mechanical energy c. low emf to high emf, or vice versa d. alternating current to direct current 23. (2 points) Which of the following options can be used to generate electricity? a. Move the circuit loop into and out of a magnetic field. b. Change the magnetic field strength around the circuit loop. c. Change the orientation of the circuit loop with respect to the magnetic field. d. all of the above 2

3 ame: ID: A 24. (2 points) Which of the following determines the maximum generated emf in an ac generator? a. the coil rotation rate or angular frequency b. the magnetic field strength, B c. the loop area, A d. all of the above 25. (2 points) A step-up transformer used on a 120 V line has 95 turns on the primary and 2850 turns on the secondary. What is the emf across the secondary? a. 30 V b V c V d V 26. (2 points) What is the path of an electron moving perpendicular to a uniform magnetic field? a. straight line b. circle c. ellipse d. parabola 27. (2 points) Under which of the following conditions is the net magnetic force on a charged particle equal to zero? a. when the particle is stationary b. when the particle is moving parallel to the magnetic field c. when the particle is not charged d. all of the above 28. (2 points) A current can be induced in a closed circuit without the use of a battery or an electrical power supply by moving the circuit through a a. high temperature field. b. gravitational field. c. magnetic field. d. nuclear field. 29. (2 points) Where is the magnitude of the magnetic field around a permanent magnet greatest? a. The magnitude is greatest close to the poles. b. The magnitude is greatest far from the poles. c. The magnitude is equal at all points on the field. d. The magnitude is greatest halfway between poles. 30. (2 points) According to the right-hand rule, if a current-carrying wire is grasped in the right hand with the thumb in the direction of the current, the four fingers will curl in the direction of a. the magnetic force,. b. the magnetic field, B. c. the current s velocity, v. d. the current s path, P. 31. (2 points) The direction of the force on a current-carrying wire in an external magnetic field is a. perpendicular to the current only. b. perpendicular to the magnetic field only. c. perpendicular to both the current and the magnetic field. d. parallel to the current and to the magnetic field. 32. (2 points) According to Lenz s law, if the applied magnetic field changes, a. the induced field attempts to keep the total field strength constant. b. the induced field attempts to increase the total field strength. c. the induced field attempts to decrease the total field strength. d. the induced field attempts to oscillate about an equilibrium value. 33. (2 points) Which conversion process is the basic function of the electric motor? a. mechanical energy to electrical energy b. electrical energy to mechanical energy c. low emf to high emf, or vice versa d. alternating current to direct current 34. (2 points) What is the path of an electron moving parallel to a uniform magnetic field? a. straight line b. circle c. ellipse d. parabola 35. (2 points) All of the following statements about magnetic field lines around a permanent magnet are true except which one? a. Magnetic field lines appear to end at the north pole of a magnet. b. Magnetic field lines have no beginning or end. c. Magnetic field lines always form a closed loop. d. In a permanent magnet, the field lines actually continue within the magnet itself. Problem 36. (5 points) The magnetic force on a wire 274 cm long is If electrons move through the wire in 1.90 s, what is the magnitude of magnetic field that is perpendicular to the wire? 3

4 ame: ID: A 37. (5 points) A step-up transformer has a 120 V rms emf in its primary coil. The primary coil has 360 turns and the secondary coil has 4800 turns. What is the maximum emf across the secondary coil? 38. (5 points) An electron moves north at a velocity of m/s and has a magnetic force of west exerted on it. If the magnetic field points upward, what is the magnitude of the magnetic field? 39. (5 points) A proton moves perpendicularly to a magnetic field that has a magnitude of T. A magnetic force of is acting on it. If the proton moves a total distance of m in the magnetic field, how long does it take for the proton to move across the magnetic field? If the magnetic force is directed north and the magnetic field is directed upward, what was the proton s velocity? 40. (5 points) A proton moves north at a velocity of m/s and has a magnetic force of east exerted on it. If the magnetic field points upward, what is the magnitude of the magnetic field? 41. (5 points) An electron is moving parallel to the Earth s surface at the equator in a direction 25 south of east. Its velocity is m/s and a magnetic force of is exerted on the electron. If the magnetic field points south at this location, what is the direction of the magnetic force on the electron and the magnitude of the magnetic field? 42. (5 points) A coil with 25 turns of wire moves through a uniform magnetic field of 1.5 T. The plane of the magnetic field is perpendicular to the plane of the coil. The coil has a cross-sectional area of 0.80 m 2. The coil enters and exits the field over 1.0 s. If the coil s resistance is 2.0 Ω, what is the induced current? 43. (5 points) The magnetic force on a straight wire 0.30 m long is The current in the wire is 15.0 A. What is the magnitude of the magnetic field that is perpendicular to the wire? 44. (5 points) A step-down transformer has turns on the primary and 95 turns on the secondary. If the rms emf across the secondary coil is 120 V, what is the maximum emf across the primary coil? 45. (5 points) A wire 48 m long carries a current of 18 A from west to east. If a magnetic field of T directed toward the south is acting on the wire, find the direction and magnitude of the magnetic force. 46. (5 points) A proton moves perpendicularly to a magnetic field that has a magnitude of T. A magnetic force of is acting on it. If the proton moves a total distance of m in the magnetic field, how long does it take for the proton to move across the magnetic field? If the magnetic force is directed north and the magnetic field is directed upward, what was the proton s velocity? 47. (5 points) A step-up transformer used on a 120 V line has 38 turns on the primary and 5163 turns on the secondary. What is the emf across the secondary? 48. (5 points) A hollow tube whose cross-sectional area is m 2 is wrapped with turns of wire. A magnetic field that changes uniformly from T to T is applied perpendicularly to the plane of the coil for s. If the current in the coil is 12.0 A, what is the coil s resistance? 49. (5 points) A wire 125 m long carries a 20.0 A current from east to west. A magnetic force of is directed downward. A proton is moving west to east with a velocity of m/s. What is the direction and magnitude of the magnetic force produced by the proton as it moves through the wire s magnetic field? 50. (5 points) A generator with a maximum output emf of 165 V is connected to a 35.0 Ω resistor. Find the maximum ac current in the circuit. 4

5 AP Physics Spring 2012 Q6 Practice Answer Section MULTIPLE CHOICE 1. AS: B V V V V : V 1 ( 115 V) V 2 ( 2.3 V) 50:1 PTS: 2 DIF: IIIA OBJ: AS: D PTS: 2 DIF: I OBJ: AS: A 35 turns A 2.0 m 2 B i 0.00 T B f 0.55 T 0.85 s θ 0.00 Substitute values into Faraday s law of magnetic induction. emf Φ M ABcosθ Acosθ B ( 35 turns) 2.0 m 2 ˆ ( 0.55 T 0.00 T) ( cos 0.00 ) ( 0.85 s) 45 V B f B i Acosθ PTS: 2 DIF: IIIB OBJ: AS: A PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: II OBJ: AS: D PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: II OBJ: AS: A PTS: 2 DIF: I OBJ: AS: C PTS: 2 DIF: I OBJ: AS: C PTS: 2 DIF: I OBJ: AS: C PTS: 2 DIF: I OBJ: ˆ 1

6 13. AS: B PTS: 2 DIF: I OBJ: AS: D PTS: 2 DIF: I OBJ: AS: D PTS: 2 DIF: I OBJ: AS: A I 10.0 A l 0.50 m 3.0 Rearrange the equation, BIl, and solve for B. B Il ( 3.0 ) ( 10.0 A) ( 0.50 m) 0.60 T PTS: 2 DIF: IIIB OBJ: AS: B PTS: 2 DIF: I OBJ: AS: C PTS: 2 DIF: II OBJ: AS: D PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: II OBJ: AS: A PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: I OBJ: AS: D PTS: 2 DIF: I OBJ: AS: D PTS: 2 DIF: I OBJ: AS: D V V 95 turns 2850 turns V 2 V turns ˆ ( 120 V) 95 turns 3600 V PTS: 2 DIF: IIIA OBJ: AS: B PTS: 2 DIF: I OBJ: AS: D PTS: 2 DIF: II OBJ: AS: C PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: I OBJ: AS: B PTS: 2 DIF: I OBJ: AS: C PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: I OBJ: AS: B PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: I OBJ: AS: A PTS: 2 DIF: I OBJ:

7 PROBLEM 36. AS: T l 274 cm 2.74 m electrons q electron C 1.90 s BIl I Q q electron electrons ˆ C/electron electrons ˆ ( 1.90 s) Rearrange the equation, BIl, and solve for B C/s 12.5 A B Il ˆ ( 12.5 A) ( 2.74 m) T PTS: 5 DIF: IIIB OBJ: AS: V V 1,rms 120 V 360 turns 4800 turns V 2,rms V 1,rms 4800 turns ˆ ( 120 V) 360 turns V Rearrange the equation, V rms V max, to solve for V max. V 2,max V 2,rms ˆ V V ( 0.707) PTS: 5 DIF: IIIB OBJ:

8 38. AS: T v m/s, north q electron C , west ˆ B q electron v ˆ C m/s ˆ T PTS: 5 DIF: IIIA OBJ: AS: s; m/s B T q proton C x m v x Substitute for v in the equation, B Rearrange the equation and solve for. v x Bq proton x qv q x ˆ ˆ T C ˆ ( m) ˆ ( m) ˆ m/s s s PTS: 5 DIF: IIIC OBJ:

9 40. AS: T v m/s, north q proton C , east ˆ B q proton v ˆ C m/s ˆ T PTS: 5 DIF: IIIA OBJ: AS: T Using the right-hand rule, is upward, since the charged particle is an electron. v m/s θ 25 south of east q electron C B direction is south. Since only the magnetic field component that is perpendicular to the electron s motion contributes to the magnetic field strength, B net Bcosθ. Substituting Bcosθ into the equation, B qv, results in Bcosθ qv. Rearrange equation, Bcosθ qv, and solve for B ˆ B q electron vcosθ ˆ C m/s ˆ T ( cos 25 ) Using the right-hand rule, is upward, since the charged particle is an electron. PTS: 5 DIF: IIIC OBJ:

10 42. AS: 15 A 25 turns A 0.80 m 2 B +1.5 T 1.0 s θ 0.00 R 2.0 Ω Use Faraday s law of magnetic induction to calculate emf. emf Φ M ABcosθ Acosθ B ( 25 turns) 0.80 m 2 ˆ ( +1.5 T) ( cos 0.00 ) 30 V ( 1.0 s) Substitute the induced emf into the definition of resistance to determine the induced current in the coil. I emf R ( 30 V) ( 2.0 Ω) 15 A PTS: 5 DIF: IIIC OBJ: AS: T I 15.0 A l 0.30 m Rearrange the equation, BIl, and solve for B. B Il ˆ ( 15.0 A) ( 0.30 m) T PTS: 5 DIF: IIIA OBJ:

11 44. AS: V V 2,rms 120 V turns 95 turns Rearrange the equation, V 2,rms V 1,rms, to solve for V 1,rms. V 1,rms V 2,rms turns ˆ ( 120 V) 95 turns V Rearrange the equation, V rms V max, to solve for V max. V 1,max V 1,rms ˆ V V ( 0.707) PTS: 5 DIF: IIIB OBJ: AS: , downward B T I 18 A l 48 m BIl ˆ T ( 18 A) ( 48 m) , downward PTS: 5 DIF: IIIA OBJ:

12 46. AS: s; m/s B T q proton C x m v x Substitute for v in the equation, B Rearrange the equation and solve for. v x Bq proton x qv q x ˆ ˆ T C ˆ ( m) ˆ ( m) ˆ m/s s s PTS: 5 DIF: IIIC OBJ: AS: V V V 38 turns 5163 turns V 2 V turns ˆ ( 120 V) 38 turns V PTS: 5 DIF: IIIA OBJ:

13 48. AS: 18.2 Ω turns A m 2 B i T B f T s θ 0.00 I 12.0 A Use Faraday s law of magnetic induction to calculate emf. emf Φ M ABcosθ Acosθ B ˆ B f B i Acosθ ˆ turns m 2 ˆ T [ T] ˆ ( cos 0.00 ) 218 V ( s) Rearrange the definition of resistance equation, I emf R R emf I 218 V 12.0 A 18.2 Ω, to find R. PTS: 5 DIF: IIIC OBJ:

14 49. AS: , upward on wire l 125 m I 20.0 A, east to west v m/s q proton C First, find the magnitude and direction of B wire. Rearrange the equation, BIl, and solve for B. B wire Il ˆ ( 20.0 A) ( 125 m) T, north Second, rearrange the equation, B, to find F qv magnetic produced by proton. produced by proton B wire q proton v ˆ T C ˆ m/s ˆ , upward PTS: 5 DIF: IIIC OBJ: AS: 4.71 A V max 165 V R 35.0 Ω I max V max R 165 V 35.0 Ω 4.71 A PTS: 5 DIF: IIIC OBJ:

15 AP Physics Spring 2012 Q6 Practice [Answer Strip] ID: A _ C 11. _ D 24. _ B 1. _ C 12. _ B 13. _ D 25. _ B 26. _ D 2. _ D 14. _ D 27. _ A 3. _ D 15. _ C 28. _ A 4. _ A 16. _ A 29. _ B 17. _ B 30. _ A 5. _ D 6. _ A 7. _ C 18. _ D 19. _ A 20. _ A 21. _ C 31. _ A 32. _ B 33. _ A 34. _ A 35. _ A 8. _ A 22. _ A 9. _ D 23. _ C 10.

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