2012 Spring Chapters 20 & 21. Magnetism Outline

Transcription

1 Tuesday February 14 Lecture: Ch 20 Introduce Electric Motor Project Lab: Uniform Magnetic Fields Homework Chapter 20 Practice Questions (25) MCAT (34) Physlet Practice Problems (9) Outline Thursday February 16 Turn in Uniform Magnetic Fields Lab Lecture: Ch 21 Electromagnetism, Lenz s Law Introduce Electromagnetic Induction Lab Homework Chapter 21 Practice Questions (25) MCAT (29) Physlet Practice Problems (10) 5 th 6 Weeks (16.5 Days) Tuesday February 21 Turn in Motor we verify it works! Turn in Electromagnetic Induction Lab Quiz Review Thursday February 23 Exam Unit 12 : Chapters 20 & 21 Homework: Chapter 23: Warmups(4) Applications (4) Chapter 24: Warmups (4) Applications (4) 48

2 Homemade Motor Project You will be making an electric motor from materials you find in the room. You may work in pairs, but you need to build your own. By the end of the period each pair needs to have a working motor. You may work with other groups to figure out how to make the motor, and you may look on the internet to get ideas. Materials you MAY use: (and I highly suggest you do!) 1 magnet, some wire, one battery. After you successfully make a motor, see what you can do to make a motor that spins faster. Your grade will be based on having a working motor, and answering the following questions. Analysis Questions: 1. What could/did you change about your motor to make it spin faster? List at least two changes and explain why they would make the motor spin faster. 2. Explain in detail why your motor works. Use physics in your description. You may need to look in your book or on the internet for help with this. Note: in detail means more than a couple of sentences. Include a diagram of your motor showing any relevant force and field vectors. 49

3 UNIFORM MAGNETIC FIELDS AP Physics B OBJECTIVES The objective of this lab is to generate and investigate controllable uniform magnetic fields. INTRODUCTION/THEORY A solenoid is a long wire wound in a closely packed helix. For points very close to a single turn of the solenoid, the magnetic properties of the current carrying wire are very similar to those of a long straight wire. The magnetic field lines close to individual wires are very nearly concentric with a direction given by the right hand rule. (Right Hand Rule: With the thumb of your right hand pointing in the direction of the wire s current, your fingers will wrap around the wire in the direction of the magnetic field lines.) The solenoid field is the vector sum of the fields set up by all of the turns that make up the solenoid. Therefore, between the individual turns of the coil, the magnetic fields tend to cancel out, while near the center the magnetic field becomes larger and more uniform. Figure #1 illustrates a loosely wound solenoid showing the position and direction of the magnetic field lines. As the solenoid is more and more tightly packed, the magnetic field near the center becomes more and more uniform, and the magnetic field outside the solenoid windings approaches zero. To obtain the best uniformity of the interior magnetic field, the length of the coil should be large compared to its diameter. By applying Ampere s law to an ideal solenoid, we can write an equation that will describe the solenoids magnetic field as a function of the current being carried by the coil and the number of turns per unit length of coil. This expression is: B oin (1) Where μ o = 4 x 10-7 H/m (permeability of free space) 50

4 I = Amp (current flowing through the solenoid) n = #/m (number of turns per unit length of the coil) Although, this equation was derived for an infinitely long solenoid, it holds quite well for actual solenoids for internal points near the center of the solenoid. Induction: A current can be induced in a wire by either moving a loop of wire in a constant magnetic field or by exposing the loop of wire to a changing magnetic field. An important law that describes the generation of an emf is Faraday s Law which states that the induced emf in the circuit is numerically equal to the rate of change of the magnetic flux through it. For a coil of many turns, Faraday s Law becomes: E=N(dΦ/dt) (2) Where N is the number of turns in the coil of wire. This equation says that the emf is the time rate of change of the magnetic flux through the coil. The magnetic flux through a coil of wire is the strength of the magnetic field perpendicular to the open surface of the coil times the area of the coil. Substituting this expression for the magnetic flux into the equation for induced emf yields: or upon expansion: E=N(d(B *A)/dt) (3) E=N((A*dB/dt)+(B*dA/dt)) (4) The first term says that we will generate an emf if our coil of constant area is exposed to a changing magnetic field. The second term says that we will generate an emf if the area of our coil changes while being held in a constant magnetic field. The total emf is the sum of both parts. Remember that the area of the coil is merely the area perpendicular to the magnetic field lines. With this in mind, the area can be changed by either physically distorting the shape of the coil or by tipping it with respect to the magnetic field lines. There is just one more piece to add to the equation for induced emf; this is Lenz s Law. This law simply states that the direction of the induced current is such that this current opposes its cause (this is very similar to inertia). This law shows up as a minus sign in the equation for Faraday s Law. APPARATUS E= -N((A*(dB/dt)+(B*dA/dt)) (5) For this experiment you will need a sensitive volt meter or an oscilloscope (the oscilloscope will provide a much more graphic image of the changing voltages than will the voltmeter), solenoid, leads, bar magnet and scrap wire. 51

5 PROCEDURES Your solenoid can be used to demonstrate the principles of induction. Connect the two leads of the coil directly to the leads of the meter. While watching the meter, bring the north pole of a magnet close to the open end of the coil. What happened to the voltage that you were monitoring? Pull the magnet away. What happened to the voltage this time? Make a note about the polarity of the voltage as the magnet is brought up to the coil and then when the magnet is withdrawn. Compare these observations with a second trial using the south pole of the bar magnet. What happens to the emf when the bar magnet is brought up to the coil very quickly? Try this while watching the volt meter. This is evidence of the first part of Faraday s Law: the area of the coil is constant and we are changing the magnetic flux through the coil. This emf is also dependent upon the speed with which this flux changes; the shorter the length of time required to change the flux, the larger the emf. Try holding the bar magnet stationary a small distance in front of the coil and then (without changing the distance between the coil and the magnet) turn the coil away from the pole of the magnet. What happens to the emf? This is evidence of the second part of Faraday s law where the area of the coil (remember, it s the area that is perpendicular to the magnetic lines of force) changes. We could also demonstrate this by changing the physical shape of the coil by pushing the sides of the coil together. You may wish to make a small coil from spare wire and try this yourself. Do not try to change the shape of your solenoid because this would damage your apparatus. ANALYSIS How did the trial using the north pole differ from the trial using the south pole? What you have been observing is evidence of Lenz s Law which says that the direction of the emf is such that it will oppose its cause (in other words, the small magnetic field produced by the emf in the coil will oppose the change in magnetic flux that generated the emf in the first place!). 52

6 Electromagnetic Induction AP Physics B OBJECTIVES In this laboratory, students will study the relationship between moving and electricity. APPARATUS Use the simulation Faraday s Electromagnetic Lab (in the Electricity,, and Circuits section) on the website to complete Lab. PROCEDURES Bar Magnet Tab General 1. Click on the Bar Magnet Tab and you should see a bar magnet and compass on the screen. Please note that the color red refers to North and white refers to South. Place the compass at the North end of the bar magnet and observe which way the red tip of the compass points. Move the compass to the South end and observe where the red tip of the compass points. What can you say about where the north (red) tip of a compass points? 2. Use your response to #1 to explain why the geographic north pole is the magnetic south pole. Pickup Coil Tab General Electromagnetic Induction 3. Set the number of loops to 1 and note what happens to the light bulb when a. The magnet is not moving and is not in the loop b. The magnet is moving and is not in the loop c. The magnet is not moving and is in the loop d. The magnet is moving and is in the loop - 4. Does the speed of the magnet affect your results to #3? If so, describe how. 5. Increase the number of loops to 3 and see if it affects your results from #3. If so, describe how. 6. Increase the loop area to 100 and see if it affects your results from #3. If so, describe how. Electromagnet Tab Is Electromagnetism Reversible? 7. You should see a battery attached to a loop of coil (an electromagnet) and a compass on the screen. Move the electromagnet around the screen and describe what the compass does. 8. Move the compass around the electromagnet in order to determine the North and South poles. Draw a picture and label the North and South Poles. 9. Change your current source from DC to AC and describe what the compass does. 10. Observe the electrons in the AC current source and compare their movement to those in the DC current source. Explain the difference between DC and AC in terms of electron movement. Transformer Tab Can We Use Electromagnetism? 11. The last tab showed us that current can create a magnetic field. Can this magnetic field generate electricity? That is, can we use electricity to generate more electricity? Move the electromagnetic back and forth and note what happens. 12. Can electricity be used to create more electricity? Explain how. 53

7 13. Change to an AC source. Note what happens while the electromagnet is not moving. Why does the light bulb light up? Do the electrons in the light bulb move as fast as the AC source? Generator Tab Putting it All Together Turn on the water faucet and describe what happens Rubric: Each question is 5 pts. 5 Complete, accurate and clear 4 Missing only 1 of above 3 Missing 2 of above 2 Incomplete but some facts 1 Confusing but some factoid correct 0 No response -1 Each Incorrect statement -1 Sentence Fragment 54

8 Multiple Choice 1. In the formula F qv B : A) F must be perpendicular to v but not necessarily to B B) F must be perpendicular to B but not necessarily to v C) v must be perpendicular to B but not necessarily to F D) all three vectors must be mutually perpendicular E) F must be perpendicular to both v and B 2. The magnetic force on a charged particle is in the direction of its velocity if: A) it is moving in the direction of the field B) it is moving opposite to the direction of the field C) it is moving perpendicular to the field D) it is moving in some other direction E) never 3. A magnetic field exerts a force on a charged particle: A) always B) never C) if the particle is moving across the field lines D) if the particle is moving along the field lines E) if the particle is at rest 4. An electron is moving north in a region where the magnetic field is south. The magnetic force exerted on the electron is: A) zero B) up C) down D) east E) west 5. A magnetic field CANNOT: A) exert a force on a charge B) accelerate a charge C) change the momentum of a charge D) change the kinetic energy of a charge E) exist 6. A hydrogen atom that has lost its electron is moving east in a region where the magnetic field is directed from south to north. It will be deflected: A) up B) down C) north D) south E) not at all 55

9 7. A beam of electrons is sent horizontally down the axis of a tube to strike a fluorescent screen at the end of the tube. On the way, the electrons encounter a magnetic field directed vertically downward. The spot on the screen will therefore be deflected: A) upward B) downward C) to the right as seen from the electron source D) to the left as seen from the electron source E) not at all 8. An electron (charge = C) is moving at m/s in the positive x direction. A magnetic field of 0.8 T is in the positive z direction. The magnetic force on the electron is: A) 0 B) N in the positive z direction C) N in the negative z direction D) N in the positive y direction E) N in the negative y direction 9. An electron travels due north through a vacuum in a region of uniform magnetic field B that is also directed due north. It will: A) be unaffected by the field B) speed up C) slow down D) follow a right-handed corkscrew path E) follow a left-handed corkscrew path 10. A uniform magnetic field is in the positive z direction. A positively charged particle is moving in the positive x direction through the field. The net force on the particle can be made zero by applying an electric field in what direction? A) Positive y B) Negative y C) Positive x D) Negative x E) Positive z 11. An electron is travelling in the positive x direction. A uniform electric field E is in the negative y direction. If a uniform magnetic field with the appropriate magnitude and direction also exists in the region, the total force on the electron will be zero. The appropriate direction for the magetic field is: A) the positive y direction B) the negative y direction C) into the page D) out of the page E) the negative x direction 56

10 12. An ion with a charge of C is in region where a uniform electric field of V/m is perpendicular to a uniform magnetic field of 0.8 T. If its acceleration is zero then its speed must be: A) 0 B) m/s C) m/s D) m/s E) any value but J. J. Thomson's experiment, involving the motion of an electron beam in mutually perpendicular E and B fields, gave the value of: A) mass of electron B) charge of electron C) Earth's magnetic field D) charge/mass ratio for electron E) Avogadro's number 14. At one instant an electron is moving in the positive x direction along the x axis in a region where there is a uniform magnetic field in the positive z direction. When viewed from a point on the positive z axis, it subsequent motion is: A) straight ahead B) counterclockwise around a circle in the xy plane C) clockwise around a circle in the xy plane D) in the positive z direction E) in the negative z direction 15. A uniform magnetic field is directed into the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius as shown. A reasonable explanation is: A) the charge is positive and slowing down B) the charge is negative and slowing down C) the charge is positive and speeding up D) the charge is negative and speeding up E) none of the above 57

11 16. An electron and a proton both each travel with equal speeds around circular orbits in the same uniform magnetic field, as shown in the diagram (not to scale). The field is into the page on the diagram. Because the electron is less massive than the proton and because the electron is negatively charged and the proton is positively charged: A) the electron travels clockwise around the smaller circle and the proton travels counterclockwise around the larger circle. B) the electron travels counterclockwise around the smaller circle and the proton travels clockwise around the larger circle C) the electron travels clockwise around the larger circle and the proton travels counterclockwise around the smaller circle D) the electron travels counterclockwise around the larger circle and the proton travels clockwise around the smaller circle E) the electron travels counterclockwise around the smaller circle and the proton travels counterclockwise around the larger circle 17. An electron and a proton are both initially moving with the same speed and in the same direction at 90 to the same uniform magnetic field. They experience magnetic forces, which are initially: A) identical B) equal in magnitude but opposite in direction C) in the same direction and differing in magnitude by a factor of 1840 D) in opposite directions and differing in magnitude by a factor of 1840 E) equal in magnitude but perpendicular to each other 18. The diagram shows a straight wire carrying a flow of electrons into the page. The wire is between the poles of a permanent magnet. The direction of the magnetic force exerted on the wire is: A) B) C) D) E) into the page 58

12 19. The diagram shows a straight wire carrying current i in a uniform magnetic field. The magnetic force on the wire is indicated by an arrow but the magnetic field is not shown. Of the following possibilities, the direction of the magnetic field is: A) to the right B) opposite the direction of F C) in the direction of F D) into the page E) out of the page 20. The figure shows the motion of electrons in a wire which is near the N pole of a magnet. The wire will be pushed: A) toward the magnet B) away from the magnet C) downwards D) upwards E) along its length 21. The figure shows a uniform magnetic field B directed to the left and a wire carrying a current into the page. The magnetic force acting on the wire is: A) toward the top of the page B) toward the bottom of the page C) toward the left D) toward the right E) zero 59

13 22. A loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal sides, each 15 cm long. A 0.7 T uniform magnetic field is parallel to the hypotenuse. The resultant magnetic force on the two sides has a magnitude of: A) 0 B) 0.21 N C) 0.30 N D) 0.41 N E) 0.51 N 23. Electrons are going around a circle in a counterclockwise direction as shown. At the center of the circle they produce a magnetic field that is: A) into the page B) out of the page C) to the left D) to the right E) zero 24. Lines of the magnetic field produced by a long straight wire carrying a current are: A) in the direction of the current B) opposite to the direction of the current C) leave the wire radially D) are circles concentric with the wire E) are lines similar to those produced by a bar magnet 25. In an overhead straight wire, the current is north. The magnetic field due to this current, at our point of observation, is: A) east B) up C) north D) down E) west 26. A wire carrying a large current i from east to west is placed over an ordinary magnetic compass. The end of the compass needle marked "N" will point: A) north B) south C) east D) west E) the compass will act as an electric motor, hence the needle will keep rotating 60

14 27. The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to: A) R B) 1/R C) 1/R 2 D) 1/R 3 E) 1/R 3/2 28. The magnetic field a distance 2 cm from a long straight current-carrying wire is T. The current in the wire is: A) 0.16 A B) 1.0 A C) 2.0 A D) 4.0 A E) 25 A 29. Two long parallel straight wires carry equal currents in opposite directions. At a point midway between the wires, the magnetic field they produce is: A) zero B) non-zero and along a line connecting the wires C) non-zero and parallel to the wires D) non-zero and perpendicular to the plane of the two wires E) none of the above 30. Two long straight current-carrying parallel wires cross the x axis and carry currents I and 3I in the same direction, as shown. At what value of x is the net magnetic field zero? A) 0 B) 1 C) 3 D) 5 E) 7 61

15 31. Two long straight wires pierce the plane of the paper at vertices of an equilateral triangle as shown below. They each carry 2 A, out of the paper. The magnetic field at the third vertex (P) has magnitude (in T): A) B) C) D) E) The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank the wires according to the magnitudes of the magnetic forces on them, from least to greatest. A) 1, 2, 3 B) 2, 1 and 3 tie C) 2 and 3 tie, then 1 D) 1 and 3 tie, then 2 E) 3, 2, A constant current is sent through a helical coil. The coil: A) tends to get shorter B) tends to get longer C) tends to rotate about its axis D) produces zero magnetic field at its center E) none of the above 34. Four long straight wires carry equal currents into the page as shown. The magnetic force exerted on wire F is: A) north B) east C) south D) west E) zero 62

16 35. Magnetic field lines inside the solenoid shown are: A) clockwise circles as one looks down the axis from the top of the page B) counterclockwise circles as one looks down the axis from the top of the page C) toward the top of the page D) toward the bottom of the page E) in no direction since B = A car travels northward at 75 km/h along a straight road in a region where Earth's magnetic field has a vertical component of T. The emf induced between the left and right side, separated by 1.7 m, is: A) 0 B) 1.8 mv C) 3.6 mv D) 6.4 mv E) 13 mv 37. The graph shows the magnitude B of a uniform magnetic field that is perpendicular to the plane of a conducting loop. Rank the five regions indicated on the graph according to the magnitude of the emf induced in the loop, from least to greatest. A) 1, 2, 3, 4 B) 2, 4, 3, 2 C) 4, 3, 1, 2 D) 1, 3, 4, 2 E) 4, 3, 2, 1 63

17 38. The circuit shown is in a uniform magnetic field that is into the page. The current in the circuit is 0.20 A. At what rate is the magnitude of the magnetic field changing: Is it increasing or decreasing?: A) zero B) 140 T/s, decreasing C) 140 T/s, increasing D) 420 T/s, decreasing E) 420 T/s, increasing 39. A changing magnetic field pierces the interior of a circuit containing three identical resistors. Two voltmeters are connected to the same points, as shown. V 1 reads 1 mv. V 2 reads: A) 0 B) 1/3 mv C) 1/2 mv D) 1 mv E) 2 mv 40. The four wire loops shown have edge lengths of either L, 2L, or 3L. They will move with the same speed into a region of uniform magnetic field B, directed out of the page. Rank them according to the maximum magnitude of the induced emf, least to greatest. A) 1 and 2 tie, then 3 and 4 tie B) 3 and 4 tie, then 1 and 2 tie C) 4, 2, 3, 1 D) 1, then 2 and 3 tie, then 4 E) 1, 2, 3, 4 64

18 41. A rectangular loop of wire is placed perpendicular to a uniform magnetic field and then spun around one of its sides at frequency f. The induced emf is a maximum when: A) the flux is zero B) the flux is a maximum C) the flux is half its maximum value D) the derivative of the flux with respect to time is zero E) none of the above 42. A copper hoop is held in a vertical east-west plane in a uniform magnetic field whose field lines run along the north-south direction. The largest induced emf is produced when the hoop is: A) rotated about a north-south axis B) rotated about an east-west axis C) moved rapidly, without rotation, toward the east D) moved rapidly, without rotation, toward the south E) moved rapidly, without rotation, toward the northwest 43. A 10 turn conducting loop with a radius of 3.0 cm spins at 60 revolutions per second in a magnetic field of 0.50 T. The maximum emf generated is: A) V B) 0.53 V C) 5.3 V D) 18 V E) 180 V 44. A copper penny slides on a horizontal frictionless table. There is a square region of constant uniform magnetic field perpendicular to the table, as shown. Which graph correctly shows the speed v of the penny as a function of time t? A) I B) II C) III D) IV E) V 65

19 45. A rod with resistance R lies across frictionless conducting rails in a constant uniform magnetic field B, as shown. Assume the rails have negligible resistance. The magnitude of the force that must be applied by a person to pull the rod to the right at constant speed v is: A) 0 B) BLv C) BLv/R D) B 2 L 2 v/r E) B 2 Lxv/R 46. As an externally generated magnetic field through a certain conducting loop increases in magnitude, the field produced at points inside the loop by the current induced in the loop must be: A) increasing in magnitude B) decreasing in magnitude C) in the same direction as the applied field D) directed opposite to the applied field E) perpendicular to the applied field 47. At any instant of time the total magnetic flux through a stationary conducting loop is less in magnitude than the flux associated with an externally applied field. This might occur because: A) the applied field is normal to the loop and increasing in magnitude B) the applied field is normal to the loop and decreasing in magnitude C) the applied field is parallel to the plane of the loop and increasing in magnitude D) the applied field is parallel to the plane of the loop and decreasing in magnitude E) the applied field is tangent to the loop 48. A rectangular loop of wire is placed midway between two long straight parallel conductors as shown. The conductors carry currents i 1 and i 2 as indicated. If i 1 is increasing and i 2 is constant, then the induced current in the loop is: A) zero B) clockwise C) counterclockwise D) depends on i 1 i 2 E) depends on i 1 + i 2 66

20 49. You push a permanent magnet with its north pole away from you toward a loop of conducting wire in front of you. Before the north pole enters the loop the current in the loop is: A) zero B) clockwise C) counterclockwise D) to your left E) to your right 50. The figure shows a bar moving to the right on two conducting rails. To make an induced current i in the direction indicated, a constant magnetic field in region A should be in what direction? A) Right B) Left C) Into the page D) Out of the page E) Impossible, cannot be done with a constant magnetic field 67

21 Answer Key 1. E 2. E 3. C 4. A 5. D 6. A 7. C 8. D 9. A 10. B 11. C 12. D 13. D 14. B 15. B 16. A 17. B 18. A 19. E 20. D 21. A 22. A 23. A 24. E 25. E 26. B 27. B 28. C 29. D 30. C 31. B 32. B 33. A 34. B 35. C 36. B 37. B 38. B 39. E 40. D 41. A 42. B 43. C 44. D 45. D 46. D 47. A 48. C 49. C 68

22 50. C 69