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# Ppa6 concep tests_ch_20

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### Ppa6 concep tests_ch_20

1. 1. ConcepTest PowerPoints Chapter 20 Physics: Principles with Applications, 6th edition Giancoli © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials.
2. 2. ConcepTest 20.1a Magnetic Force I A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) downwards of the magnetic force? 4) to the right 5) to the left x x x x x x v x x x x x x x x x xq x x
3. 3. ConcepTest 20.1a Magnetic Force I A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) downwards of the magnetic force? 4) to the right 5) to the left Using the right-hand rule, you can see that the magnetic force is x x x x x x directed to the left. Remember v x x x x x x that the magnetic force must be perpendicular to BOTH the B field x xFx xq x x and the velocity.
4. 4. ConcepTest 20.1b Magnetic Force II A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) downwards of the magnetic force? 4) upwards 5) to the left x x x x x x x x x x vx x q x x x x x x
5. 5. ConcepTest 20.1b Magnetic Force II A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) downwards of the magnetic force? 4) upwards 5) to the left Using the right-hand rule, you can see that the magnetic force is x x x x x x directed upwards. Remember F x x x x vx x that the magnetic force must be q perpendicular to BOTH the B field x x x x x x and the velocity.
6. 6. ConcepTest 20.1c Magnetic Force III A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) zero of the magnetic force? 4) to the right 5) to the left   v  q 
7. 7. ConcepTest 20.1c Magnetic Force III A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) zero of the magnetic force? 4) to the right 5) to the left Using the right-hand rule, you can  see that the magnetic force is directed into the page. Remember  v that the magnetic force must be  perpendicular to BOTH the B field  q F and the velocity. 
8. 8. ConcepTest 20.1d Magnetic Force IV A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) zero of the magnetic force? 4) to the right 5) to the left  v   q 
9. 9. ConcepTest 20.1d Magnetic Force IV A positive charge enters a 1) out of the page uniform magnetic field as 2) into the page shown. What is the direction 3) zero of the magnetic force? 4) to the right 5) to the left The charge is moving parallel to  the magnetic field, so it does not v experience any magnetic force.  Remember that the magnetic force  q is given by: F = v B sin(q) . F=0 
10. 10. ConcepTest 20.2 Atomic Beams A beam of atoms enters x x x x x x x x x1 x x x a magnetic field region. x x x x x x x x x x x x 2 What path will the x x x x x x x x x x x x atoms follow? x x x x x x x x x x x x x x x x x x x x x x x x 3 4 x x x x x x x x x x x x
11. 11. ConcepTest 20.2 Atomic Beams A beam of atoms enters x x x x x x x x x1 x x x a magnetic field region. x x x x x x x x x x x x 2 What path will the x x x x x x x x x x x x atoms follow? x x x x x x x x x x x x x x x x x x x x x x x x 3 4 x x x x x x x x x x x x Atoms are neutral objects whose net charge is zero. Thus they do not experience a magnetic force. Follow-up: What charge would follow path #3? What about path #1?
12. 12. ConcepTest 20.3 Magnetic Field 1) + y A proton beam enters into a magnetic field region as shown 2) – y below. What is the direction of 3) + x the magnetic field B? 4) + z (out of page) 5) – z (into page) y x
13. 13. ConcepTest 20.3 Magnetic Field 1) + y A proton beam enters into a magnetic field region as shown 2) – y below. What is the direction of 3) + x the magnetic field B? 4) + z (out of page) 5) – z (into page) The picture shows the force acting in the +y direction. Applying the y right-hand rule leads to a B field that points into the page. The B field must be out of the plane x because B  v and B  F. Follow-up: What would happen to a beam of atoms?
14. 14. ConcepTest 20.4a Mass Spectrometer I x x x x x x x x x x x x Two particles of the same mass x x x x x x x x x x x x enter a magnetic field with the x x x x x x x x x x x x same speed and follow the paths x x x x x x x x x x x x shown. Which particle has the x x x x x x x x x x x x bigger charge? x x x x x x x x x x x x 1 2 3) both charges are equal 4) impossible to tell from the picture
15. 15. ConcepTest 20.4a Mass Spectrometer I x x x x x x x x x x x x Two particles of the same mass x x x x x x x x x x x x enter a magnetic field with the x x x x x x x x x x x x same speed and follow the paths x x x x x x x x x x x x shown. Which particle has the x x x x x x x x x x x x bigger charge? x x x x x x x x x x x x 1 2 3) both charges are equal 4) impossible to tell from the picture The relevant equation for us is: mv R According to this equation, the qB bigger the charge, the smaller the radius. Follow-up: What is the sign of the charges in the picture?
16. 16. ConcepTest 20.4b Mass Spectrometer II A proton enters a uniform 1) it increases magnetic field that is 2) it decreases perpendicular to the 3) it stays the same proton’s velocity. What happens to the kinetic 4) depends on the velocity direction energy of the proton? 5) depends on the B field direction x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
17. 17. ConcepTest 20.4b Mass Spectrometer II A proton enters a uniform 1) it increases magnetic field that is 2) it decreases perpendicular to the 3) it stays the same proton’s velocity. What happens to the kinetic 4) depends on the velocity direction energy of the proton? 5) depends on the B field direction x x x x x x x x x x x x The velocity of the proton x x x x x x x x x x x x changes direction but the x x x x x x x x x x x x magnitude (speed) doesn’t x x x x x x x x x x x x change. Thus the kinetic energy stays the same. x x x x x x x x x x x x
18. 18. ConcepTest 20.5 Velocity Selector What direction would a B field 1) up (parallel to E ) have to point for a beam of 2) down (antiparallel to E ) electrons moving to the right to 3) into the page go undeflected through a region 4) out of the page where there is a uniform electric 5) impossible to accomplish field pointing vertically upward? E B=? electrons v
19. 19. ConcepTest 20.5 Velocity Selector What direction would a B field 1) up (parallel to E ) have to point for a beam of 2) down (antiparallel to E ) electrons moving to the right to 3) into the page go undeflected through a region 4) out of the page where there is a uniform electric 5) impossible to accomplish field pointing vertically upward? Without a B field, the electrons feel an E electric force downwards. In order to compensate, the magnetic force has to point upwards. Using the right-hand B=? rule and the fact that the electrons are negatively charged leads to a B field electrons v pointing out of the page.
20. 20. ConcepTest 20.6a Magnetic Force on a Wire I 1) left A horizontal wire carries a current 2) right and is in a vertical magnetic field. 3) zero What is the direction of the force 4) into the page on the wire? 5) out of the page I B
21. 21. ConcepTest 20.6a Magnetic Force on a Wire I 1) left A horizontal wire carries a current and is in a vertical magnetic field. 2) right What is the direction of the force on 3) zero the wire? 4) into the page 5) out of the page Using the right-hand rule, we see that the magnetic force must point out of the page. I Since F must be perpendicular to both I and B, you should realize that F cannot be in the B plane of the page at all.
22. 22. ConcepTest 20.6b Magnetic Force on a Wire II A horizontal wire carries a current 1) left 2) right and is in a vertical magnetic field. 3) zero What is the direction of the force 4) into the page on the wire? 5) out of the page I B
23. 23. ConcepTest 20.6b Magnetic Force on a Wire II A horizontal wire carries a current 1) left and is in a vertical magnetic field. 2) right What is the direction of the force on 3) zero the wire? 4) into the page 5) out of the page I When the current is parallel to the magnetic field lines, the force on the wire is zero. B
24. 24. ConcepTest 20.7a Magnetic Force on a Loop I 1) + x A rectangular current loop is 2) + y in a uniform magnetic field. 3) zero What is the direction of the 4) - x net force on the loop? 5) - y z B y x
25. 25. ConcepTest 20.7a Magnetic Force on a Loop I 1) + x A rectangular current loop is 2) + y in a uniform magnetic field. 3) zero What is the direction of the 4) - x net force on the loop? 5) - y Using the right-hand rule, we find that each of the four wire segments will experience a force outwards from the center of the loop. Thus, the forces B z of the opposing segments cancel, so the net force is zero. y x
26. 26. ConcepTest 20.7b Magnetic Force on a Loop II 1) move up If there is a current in 2) move down the loop in the direction 3) rotate clockwise shown, the loop will: 4) rotate counterclockwise 5) both rotate and move B field out of North B field into South N S S N
27. 27. ConcepTest 20.7b Magnetic Force on a Loop II 1) move up If there is a current in 2) move down the loop in the direction 3) rotate clockwise shown, the loop will: 4) rotate counterclockwise 5) both rotate and move Look at the North Pole: here the F magnetic field points to the right and the current points out of the page. N S The right-hand rule says that the force must point up. At the south pole, the same logic leads to a downward force. F Thus the loop rotates clockwise.
28. 28. ConcepTest 20.8a Magnetic Field of a Wire I If the currents in these wires have 1) direction 1 the same magnitude, but opposite 2) direction 2 3) direction 3 directions, what is the direction of 4) direction 4 the magnetic field at point P? 5) the B field is zero 1 P 4 2 3
29. 29. ConcepTest 20.8a Magnetic Field of a Wire I If the currents in these wires have 1) direction 1 the same magnitude, but opposite 2) direction 2 3) direction 3 directions, what is the direction of 4) direction 4 the magnetic field at point P? 5) the B field is zero 1 P Using the right-hand rule, we can sketch the B fields due 4 2 to the two currents. Adding 3 them up as vectors gives a total magnetic field pointing downward.
30. 30. ConcepTest 20.8b Magnetic Field of a Wire II Each of the wires in the figures 1) arrangement 1 below carry the same current, either into or out of the page. 2) arrangement 2 In which case is the magnetic 3) arrangement 3 field at the center of the square 4) same for all greatest? 1 B=? 2 B=? 3 B=?
31. 31. ConcepTest 20.8b Magnetic Field of a Wire II Each of the wires in the figures 1) arrangement 1 below carry the same current, either into or out of the page. 2) arrangement 2 In which case is the magnetic 3) arrangement 3 field at the center of the square 4) same for all greatest? 1 2 3
32. 32. ConcepTest 20.9a Field and Force I A positive charge moves parallel 1) + z (out of page) to a wire. If a current is suddenly 2) - z (into page) turned on, which direction will the 3) + x force act? 4) - x 5) - y y +q I x z
33. 33. ConcepTest 20.9a Field and Force I A positive charge moves parallel 1) + z (out of page) to a wire. If a current is suddenly 2) - z (into page) turned on, which direction will the 3) + x force act? 4) - x 5) - y Using the right-hand rule to determine the magnetic field produced by the wire, we find that at the position of the charge +q (to the left of the wire) the B field points y out of the page. Applying the right-hand +q I rule again for the magnetic force on the charge, we find that +q experiences a force in the +x direction. x z
34. 34. ConcepTest 20.9b Field and Force II Two straight wires run parallel to 1) toward each other each other, each carrying a current in the direction shown 2) away from each other below. The two wires experience 3) there is no force a force in which direction?
35. 35. ConcepTest 20.9b Field and Force II Two straight wires run parallel to 1) toward each other each other, each carrying a current in the direction shown 2) away from each other below. The two wires experience 3) there is no force a force in which direction? The current in each wire produces a magnetic field that is felt by the current of the other wire. Using the right-hand rule, we find that each wire experiences a force toward the other wire (i.e., an attractive force) when the currents are parallel (as shown). Follow-up: What happens when one of the currents is turned off?
36. 36. ConcepTest 20.10 Current Loop 1) left What is the direction of the 2) right magnetic field at the center 3) zero (point P) of the square loop 4) into the page of current? 5) out of the page I P
37. 37. ConcepTest 20.10 Current Loop 1) left What is the direction of the 2) right magnetic field at the center 3) zero (point P) of the square loop 4) into the page of current? 5) out of the page Use the right-hand rule for each wire segment to find that each I segment has its B field pointing out of the page at point P. P