PHYSICS 212 WORKBOOK

Transcription

1 PHYSICS 212 CHAPTER 19 MAGNETISM WORKBOOK STUDENT S FULL NAME (By placing your name above and submitting this for credit you are affirming this to be predominantly your own work.) / / DATE DUE INSTRUCTIONS 1. Turn this workbook in on time for credit, even if it is not complete. (No credit if late.) 2. Complete this workbook neatly. Do not write in ink so that corrections can be made. (Credit will be lost if this is turned in messy.) 3. Complete the chapter outline section as early as possible. Don t wait for the due date to be assigned to start. 4. Complete the sections in sequence. 5. Study and learn definitions of terms, physical quantities, units, principles, and basic equations before attempting problems. 6. You may work on this with other students but do not copy another student s workbook or let a student copy this workbook. Do not copy from other sources either. 7. Wherever possible, include diagrams in your solutions. Diagrams are required. 8. Keep this workbook after it is graded and returned to you. 9. Using the answer key, redo all questions and problems until you can answer them all correctly by yourself without help. 10. Use the workbook to learn the general problem-solving strategy rather than how individual problems are solved. 11. Questions marked (Basic) should be answered by students without the need for additional assistance. -1-

2 PHYSICS 212 CHAPTER 19 OUTLINE MAGNETISM 19-1 Magnets Complete the following: 1. Magnetic fields affect and moving charges produce. 2. The ends of a bar magnet are called. And are identified as the and the. 3. Like poles and unlike poles. Compare and contrast soft and hard magnetic materials. What symbol is used to designate a magnetic field? Draw a sketch of a bar magnet. (fig 19.2) 19.2 EARTH S MAGNETIC FIELD If you were sent on a job to Earth s magnetic south pole, where would you go geographically? Define: magnetic dip angle: magnetic declination: -2-

3 19.3 MAGNETIC FIELDS What are the SI units of the magnetic field? What are the cgs units of the magnetic field and the conversion factor between cgs and SI units? Under what condition will th force on a moving charge in a magnetic field be maximum? Under what two different conditions can a charged particle be in a magnetic field and not experience any force? What is the name of the rule used to determine the direction of the force on a moving charge in a magnetic field? 19.4 MAGNETIC FORCE ON A CURRENT-CARRYING CONDUCTOR Write equation 19.6 listing and naming all variables along with their SI units. A current is directed to the east inside an apparatus in which there is an upward magnetic field. In what direction is the current displaced? 19.5 TORQUE ON A CURRENT LOOP AND ELECTRIC MOTORS Does the torque on a current loop depend on the shape of the loop? -3-

4 How should a current loop be positioned so it has no torque on it in a magnetic field? Define the magnetic moment of a coil. What symbol is used and what are the SI units? Describe the following parts of a DC motor: commutator: brushes: 19.6 MOTION OF A CHARGED PARTICLE IN A MAGNETIC FIELD When a charged particle is made to move in a direction perpendicular to a magnetic field what path will it follow? Describe the motion of a charged particle that has velocity components both perpendicular and parallel to a magnetic field? 19.7 MAGNETIC FIELD OF A LONG, STRAIGHT WIRE AND AMPÈRE S LAW What is the permeability of free space, its symbol, numeric value, and SI units? How does the magnetic field strength of the field produced by a long, current carrying wire vary with distance from the wire? State Ampère s Law. -4-

5 19.8 MAGNETIC FORCE BETWEEN TWO PARALLEL CONDUCTORS How would you characterize the force between to parallel wires carrying current in the same direction? Define the SI units Ampere and Coulomb MAGNETIC FIELDS OF CURRENT LOOPS AND SOLENOIDS Sketch the magnetic field of a current loop (fig 19.30). Show the direction of the current in the wire. Sketch the magnetic field os a solenoid (fig19.32). Show the direction of the current in the wire MAGNETIC DOMAINS What causes magnetism in materials? What are magnetic domains? -5-

6 Describe ferromagnetic materials. What determines if a magnetic material retains its magnetism? -6-

7 PHYSICS 212 CHAPTER 19 MULTIPLE CHOICE QUESTIONS MAGNETISM Multiple Choice Identify the letter of the choice that best completes the statement or answers the question. 1. Electrical charges and magnetic poles have many similarities, but one difference is: A. opposite magnetic poles repel. B. one magnetic pole cannot create magnetic poles in other materials. C. a magnetic pole cannot be isolated. D. magnetic poles do not produce magnetic fields. E. magnetic poles produce only alternating fields. (Basic) 2. Which of the following is not a hard magnetic material? A. iron D. neodymium B. cobalt E. both b and c C. nickel (Basic) 3. Geophysicists today generally attribute the existence of the Earth's magnetic field to which of the following? A. convection currents within the liquid interior B. iron ore deposits in the crust C. nickel-iron deposits in the crust D. solar flares E. iron-cobalt deposits in the crust (Basic) 4. The term magnetic declination refers to which of the following? A. angle between Earth's magnetic field and Earth's surface B. Earth's magnetic field strength at the equator C. tendency for Earth's field to reverse itself D. angle between directions to true north and magnetic north E. angle between Earth's magnetic field and Earth's rotational axis (Basic) 5. The magnetic field of the Earth is believed responsible for which of the following? A. deflection of both charged and uncharged cosmic rays B. deflection of charged cosmic rays C. ozone in the upper atmosphere D. solar flares E. deflection of uncharged cosmic rays (Basic) 6. The magnetic pole of the Earth nearest the geographic North Pole corresponds to which of the following? A. a magnetic north pole C. a magnetic arctic pole B. a magnetic south pole D. a magnetic antarctic pole (Basic) -7-

8 7. The force on a charged particle created by its motion in a magnetic field is maximum at what angle between the particle velocity and field? A. zero D. 45 B. 180 E. 135 C. 90 (Basic) 8. Assume that a uniform magnetic field is directed into this page. If an electron is released with an initial velocity directed from the bottom edge to the top edge of the page, which of the following describes the direction of the resultant force acting on the electron? A. out of the page D. into the page B. to the right E. from top edge to bottom edge of the page C. to the left 9. A proton moves across the Earth's equator in a northeasterly direction. At this point the Earth's magnetic field has a direction due north and is parallel to the surface. What is the direction of the force acting on the proton at this instant? A. toward the northwest D. toward the northeast B. out of the Earth's surface E. toward the southwest C. into the Earth's surface 10. Different units can be used to measure the same physical quantity, differing only by some multiplicative factor. The cgs unit for magnetic field, the gauss, is equal to tesla. A B C. 0.5 D. 4 E. These units do not measure the same physical quantity. 11. If a proton is released at the equator and falls toward the Earth under the influence of gravity, the magnetic force on the proton will be toward the: A. north. C. east. B. south. D. west. 12. A stationary positive charge +Q is located in a magnetic field B, which is directed toward the right as indicated. The direction of the magnetic force on Q is: A. toward the right. D. toward the left. B. up. E. There is no magnetic force. C. down. -8-

9 13. There is a magnetic force on a particle. It is possible that the particle is: A. uncharged. B. stationary. C. moving in the direction of the magnetic field. D. not part of a wire. E. moving opposite the direction of the magnetic field. (Basic) 14. Which of the following devices makes use of an electromagnet? A. loudspeaker D. both A and B B. galvanometer E. none of the above C. gyrocompass 15. The direction of the force on a current carrying wire located in an external magnetic field is which of the following? A. perpendicular to the current D. Both choices A and B are valid. B. perpendicular to the field E. None of the above are valid. C. parallel to the wire (Basic) 16. A circular current loop is placed in an external magnetic field. How is the torque related to the radius of the loop? A. directly proportional to radius B. inversely proportional to radius C. directly proportional to radius squared D. inversely proportional to radius squared E. directly proportional to square root of radius (Basic) 17. Magnetism had been a known phenomenon for some time before its relation to electric currents was found. That a current in a wire produces a magnetic field was discovered by: A. Maxwell. D. Tesla. B. Ampere. E. Faraday. C. Oersted. (Basic) 18. A current in a long, straight wire produces a magnetic field. The magnetic field lines: A. go out from the wire to infinity. D. form circles that go around the wire. B. come in from infinity to the wire. E. are parallel to the wire. C. form circles that pass through the wire. (Basic) 19. Two parallel wires are separated by 0.25 m. Wire A carries 5.0 A and Wire B carries 10 A, both currents in the same direction. The force on 0.80 m of Wire A is: A. half that on 0.80 m of wire B. D. away from Wire B. B. one-fourth that on 0.80 m of wire B. E. one-eighth that on 0.80 m of wire B. C. toward Wire B. (Basic) -9-

10 20. A current in a solenoid coil creates a magnetic field inside that coil. The field strength is directly proportional to: A. the solenoid area. D. Both A and B are valid choices. B. the current. E. None of the above choices are valid. C. the solenoid diameter. (Basic) 21. A current in a coil with N turns creates a magnetic field at the center of that loop. The field strength is directly proportional to: A. number of turns in the loop. D. Both choices A and B are valid. B. current strength. E. None of the above are valid. C. length of the coil. (Basic) 22. The magnetic domains in a non-magnetized piece of iron are characterized by which orientation? A. parallel to the magnetic axis B. anti-parallel (opposite direction) to the magnetic axis C. random D. perpendicular to the magnetic axis E. any of the above is possible. (Basic) 23. When an electromagnet has an iron core inserted, what happens to the strength of the magnet? A. It increases. B. It remains the same. C. It decreases. D. Since it depends on the metal used in the wires of the electromagnet, any of the above. (Basic) -10-

11 PHYSICS 212 CHAPTER 19 ADDITIONAL QUESTIONS MAGNETISM 1. Calculate the net force on a dust particle moving in Earth s magnetic field. The magnetic field is horizontal and directed southward with a magnitude of 0.5 G. The dust particle has a mass of 0.05 grams, a charge of -4e and is traveling horizontally eastward at 15 m/s. (Basic) 2. What is the force on a 50m length of wire carrying 15 A in a vertically upward magnetic field of 2 T? (Basic) -11-

12 3. A coil is made of 15 loops of wire with a radius of 30 cm. If it carries a current of 5 A, what is its magnetic dipole moment? Add the magnetic dipole moment to the diagram. (Basic) 4. Wire 1 lies along the y-axis with 2.5 A of current flowing in the +y direction. Wire 2 lies along the x-axis with 3.75 A of current flowing in the -x direction. What is the net magnetic field due to these currents at the point P located at coordinates (5 cm, 6 cm)? -12-

13 5. Calculate the magnetic field inside a 40 cm long solenoid if the solenoid current is 3.0 A and the there are 300 turns of wire per cm. Indicate the direction of the magnetic field also. (Basic) 6. In some region of space a uniform magnetic field of 3.5 T points to the right. A microsphere of charge -4nC and mass 1.0 mg moves in this field with a speed of m/s initially directed upward and perpendicular to the field. Draw this magnetic field and the path the microsphere will follow. What is the radius of this path? -13-

14 7. Suppose the coil (loop) of problem 3 is placed in the magnetic field of problem 6. The plane of the coil makes a 35 degree angle with the magnetic field. Calculate the magnitude of the torque acting on the loop and specify how the loop will respond to this torque. 8. A long wire carrying 7.0 A in the +y direction lies along the y-axis. Next to the wire, in the x-y plane is a square loop whose sides are 5 cm long and is made with 15 turns of wire. The left edge of the loop is 7 cm from the long wire. Compute the net force on the loop. -14-

15 PHYSICS 212 CHAPTER 19 PROBLEMS MAGNETISM PRACTICE PROBLEMS: 3, 4, 10, 11, 12, 15, 19, 20, 23, 27, 34, 39, 41, 44, 47, 61 HOMEWORK PROBLEMS: 1, 7, 13, 17, 29, 32, 40, 46, 49, 58 [NOTE: Problem numbers in italic are included in the student study guide; Problem numbers underlined are online at 1. An electron gun fires electrons into a magnetic field directed straight downward. Find the direction of the force exerted by the field on an electron for each of the following directions of the electron s velocity: (a) horizontal and due north; (b) horizontal and 30 west of north; (c) due north, but at 30 below the horizontal; (d) straight upward. (Remember that an electron has a negative charge.) (Basic) 7. What velocity would a proton need to circle Earth km above the magnetic equator, where Earth s magnetic field is directed horizontally north and has a magnitude of T? -15-

16 13. In Figure P19.3, assume that in each case the velocity vector shown is replaced with a wire carrying a current in the direction of the velocity vector. For each case, find the direction of the magnetic field that will produce the magnetic force shown. (Basic) Figure P A wire with a mass of 1.00 g/cm is placed on a horizontal surface with a coefficient of friction of The wire carries a current of 1.50 A eastward and moves horizontally to the north. What are the magnitude and the direction of the smallest vertical magnetic field that enables the wire to move in this fashion? -16-

17 29. Figure P19.29a is a diagram of a device called a velocity selector, in which particles of a specific velocity pass through undeflected while those with greater or lesser velocities are deflected either upwards or downwards. An electric field is directed perpendicular to a magnetic field, producing an electric force and a magnetic force on the charged particle that can be equal in magnitude and opposite in direction (Fig. P19.29b) and hence cancel. Show that particles with a speed of v = E/B will pass through the velocity selector undeflected. Figure P

18 32. A mass spectrometer is used to examine the isotopes of uranium. Ions in the beam emerge from the velocity selector at a speed of m/s and enter a uniform magnetic field of T directed perpendicularly to the velocity of the ions. What is the distance between the impact points formed on the photographic plate by singly charged ions of 235 U and 238 U? -18-

19 40. The two wires in Figure P19.40 carry currents of 3.00 A and 5.00 A in the direction indicated. (a) Find the direction and magnitude of the magnetic field at a point midway between the wires. (b) Find the magnitude and direction of the magnetic field at point P, located 20.0 cm above the wire carrying the 5.00-A current. Figure P

20 46. In Figure P19.46, the current in the long, straight wire is I 1 = 5.00 A, and the wire lies in the plane of the rectangular loop, which carries 10.0 A. The dimensions shown are c = m, a = m, and = m. Find the magnitude and direction of the net force exerted by the magnetic field due to the straight wire on the loop. -20-

21 49. A single-turn square loop of wire 2.00 cm on a side carries a counterclockwise current of A. The loop is inside a solenoid, with the plane of the loop perpendicular to the magnetic field of the solenoid. The solenoid has 30 turns per centimeter and carries a counterclockwise current of 15.0 A. Find the force on each side of the loop and the torque acting on the loop. -21-

22 58. Two circular loops are parallel, coaxial, and almost in contact 1.00 mm apart (Fig. P19.58). Each loop is 10.0 cm in radius. The top loop carries a clockwise current of 140 A. The bottom loop carries a counterclockwise current of 140 A. (a) Calculate the magnetic force that the bottom loop exerts on the top loop. (b) The upper loop has a mass of kg. Calculate its acceleration, assuming that the only forces acting on it are the force in part (a) and its weight. [Hint: The distance between the loops is small in comparison to their radius of curvature, so the loops may be treated as long, straight parallel wires.] Figure P