1. Magnetism Physics 102: Lecture 08
Today’s lecture will cover Textbook Sections
Physics 102: Lecture 8, Slide 1 1

2. Magnets are Cool! Lets Break it! North Pole and South Pole+ -
North Pole and South Pole
Opposites Attract
Likes Repel
Magnetic Field Lines
Arrows give direction
Density gives strength
Looks like dipole!
Lets Break it!
Physics 102: Lecture 8, Slide 2

3. Field Lines of Bar Magnet
S N
Make sure to tell students to draw lines through on their handout
Magnetic field lines don’t start or stop.
There are no magnetic charges (monopoles)
Physics 102: Lecture 8, Slide 3

4. S N 3 2 1
Preflight 8.1
Which drawing shows the correct field lines for a bar magnet?
(1)
(2)
(3)
Physics 102: Lecture 8, Slide 4

5. 1
Preflight 8.1
Which drawing shows the correct field lines for a bar magnet?
S N
(1)
(2)
(3) 2
S N
Magnetic field lines are continuous
Arrows go from N to S outside the magnet (S to N inside). 3
S N
Physics 102: Lecture 8, Slide 5

6. Comparison: Electric Field Lines vs. Magnetic Field Lines
Similarities
Density gives strength
Arrow gives direction
Leave +, North
Enter -, South
Differences
Start/Stop on electric charge
No Magnetic Charge, lines are continuous!
FYI
x x x x x x x INTO Page
• • • • • • • • • OUT of Page
Physics 102: Lecture 8, Slide 6

7. No Magnetic Charges
Magnetic Fields are created by moving electric charge!
Where is the moving charge?
Orbits of electrons about nuclei
Intrinsic “spin” of electrons (more important effect)
Physics 102: Lecture 8, Slide 7

8. Direction of Magnetic Force on Moving Charges
Velocity B Force
out of page right
out of page left
out of page up
out of page down
Do this demo with crt and bar magnet. I usually say CRT has positive charge, because – sign in direction is tough.
Might find better drawing of hand here. v B F
Right Hand Rule
Thumb ___ , Fingers ___ , Palm ___
Physics 102: Lecture 8, Slide 8

9. Direction of Magnetic Force on Moving Charges
Velocity B Force
out of page right up
out of page left down
out of page up
1) Up 2) Down 3) Right 4) Left ) Zero
Do this demo with crt and bar magnet. I usually say CRT has positive charge, because – sign in direction is tough.
Might find better drawing of hand here. v B F
Right Hand Rule
Thumb v, Fingers B, palm F
Physics 102: Lecture 8, Slide 9

10. Direction of Magnetic Force on Moving Charges
Velocity B Force
out of page right up
out of page left down
out of page up
out of page down
left
right
Right Hand Rule B
Thumb v, Fingers B, palm F v F
Negative charge has opposite F!
Physics 102: Lecture 8, Slide 10

11. Preflight 8.3
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the direction of the force on the particle just as it enters region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 11

12. Preflight 8.3
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the direction of the force on the particle just as it enters region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page
Particle is moving straight upwards then veers to the right.
Physics 102: Lecture 8, Slide 12

13. Preflight 8.4
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 13

14. Preflight 8.4
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page _
v (thumb) points up, F(palm) points right: so B(fingers) must point out.
Physics 102: Lecture 8, Slide 14

15. ACT: 2 Chambers
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 2?
1) up
2) down
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 15

16. ACT: 2 Chambers
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 2?
1) up
2) down
3) left
4) right
5) into page
6) out of page
v (thumb) points right, F(palm) points up, B(fingers) point in.
Physics 102: Lecture 8, Slide 16

17. Magnitude of Magnetic Force on Moving Charges
The magnetic force on a charge depends on the magnitude of the charge, its velocity, and the magnetic field.
F = q v B sin(q)
Direction from RHR
Thumb (__), fingers (__), palm (__)
Note if v is parallel to B then F = 0 V q B
Physics 102: Lecture 8, Slide 17

18. ACT: Moving Charges
The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field.
1) Which particle experiences the greatest magnetic force?
1) 1 2) 2 ) 3 4) All Same
2) The force on particle 3 is in the same direction as the force on particle 1.
1) True 2) False B 3 2 1
Physics 102: Lecture 8, Slide 18

19. ACT: Moving Charges F = q v B sin(q) B into page!
The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field.
1) Which particle experiences the greatest magnetic force?
1) 1 2) 2 3) 3 4) All Same
2) The force on particle 3 is in the same direction as the force on particle 1.
1) True 2) False
F = q v B sin(q) B
Thumb (v), fingers (B), palm (F)
into page! 3 2 1
Physics 102: Lecture 8, Slide 19

20. Comparison Electric vs. Magnetic
Electric Magnetic
Source: Charges Moving Charges
Act on: Charges Moving Charges
Magnitude: F = qE F = q v B sin(q)
Direction: Parallel to E Perpendicular to v,B
Physics 102: Lecture 8, Slide 20

21. Velocity Selector Example
Determine magnitude and direction of magnetic field such that a positively charged particle with initial velocity v travels straight through and exits the other side. E v
What direction should B point if you want to select negative charges?
1) Into Page 2) Out of page 3) Left 4) Right
Physics 102: Lecture 8, Slide 21

22. Velocity Selector Example
Determine magnitude and direction of magnetic field such that a positively charged particle with initial velocity v travels straight through and exits the other side. FB E v FE
Electric force is down, so need magnetic force up.
By RHR, B must be into page
For straight line, need |FE |= |FB |
q E= q v B sin(90)
B = E/v
What direction should B point if you want to select negative charges?
1) Into Page 2) Out of page 3) Left 4) Right
FE would be up so FB must be down.
Physics 102: Lecture 8, Slide 22

23. Motion of Q in uniform B field
Force is perpendicular to B,v
B does no work! (W=F d cos q )
Speed is constant (W=D K.E. )
Circular motion
Force is perpendicular to B,v
x x x x x x x v
Solve for R:
Uniform B into page
Physics 102: Lecture 8, Slide 23

24. Motion of Q in uniform B field
Force is perpendicular to B,v
B does no work! (W=F d cos q )
Speed is constant (W=D K.E. )
Circular motion
Force is perpendicular to B,v
x x x x x x x v R F v F v F v F v F v F
Solve for R:
Uniform B into page
Physics 102: Lecture 8, Slide 24

25. Preflight 8.6 1) v2 < v1 2) v2 = v1 3) v2 > v1
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the speed of the particle when it leaves chamber 2?
1) v2 < v1
2) v2 = v1
3) v2 > v1
Physics 102: Lecture 8, Slide 25

26. Preflight 8.6 1) v2 < v1 2) v2 = v1 3) v2 > v1
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
What is the speed of the particle when it leaves chamber 2?
1) v2 < v1
2) v2 = v1
3) v2 > v1
Magnetic force is always perpendicular to velocity, so it changes direction, not speed of particle. 43
Physics 102: Lecture 8, Slide 26

27. Preflight 8.8
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
Compare the magnitude of the magnetic field in chambers 1 and 2
1) B1 > B2
2) B1 = B2.
3) B1 < B2
Physics 102: Lecture 8, Slide 27

28. Preflight 8.8
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = +25 mC
Compare the magnitude of the magnetic field in chambers 1 and 2
1) B1 > B2
2) B1 = B2.
3) B1 < B2
Larger B, greater force, smaller R
Physics 102: Lecture 8, Slide 28

29. Preflight 8.9
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = ?? mC
A second particle with mass 2m enters the chamber and follows the same path as the particle with mass m and charge q=25 mC. What is its charge?
1) Q = 12.5 mC
2) Q = 25 mC
3) Q = 50 mC
Physics 102: Lecture 8, Slide 29

30. Preflight 8.9
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 1 2
v = 75 m/s
q = ?? mC
A second particle with mass 2m enters the chamber and follows the same path as the particle with mass m and charge q=25 mC. What is its charge?
1) Q = 12.5 mC
2) Q = 25 mC
3) Q = 50 mC
Physics 102: Lecture 8, Slide 30

31. See you later!
Read Sections
Physics 102: Lecture 8, Slide 31