1. Today’s Lecture: Magnetism
Physics 102: Lecture 08
Today’s Lecture: Magnetism
Magnetic fields
Magnetic force on charged particles 1

2. Magnets North Pole and South Pole Magnetic Field Lines+ -
North Pole and South Pole
Opposites Attract
Likes Repel
Magnetic Field Lines
Arrows give direction
Density gives strength
Looks like dipole! 15

3. The Earth is a Magnet

5. 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)

6. Magnetic Fields and Forces
Magnetic fields exert forces on moving charge
We will discuss the magnitude of the force shortly
Direction of force is given by a “right-hand rule”

7. Direction of Magnetic Force on Moving Charges
Magnetic force is perpendicular to both B and v
Right-hand rule:
Point thumb of right hand in v direction
Point fingers of right hand in B direction
Then out-of-palm points
in the direction of F for + charge
opposite to F for – charge
Note:
different versions by different
books etc.
Need a prop to show this.

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
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.
ACT:
1) Up 2) Down 3) Right 4) Left ) Zero

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

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

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

14. 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?
A) down
B) left
C) right
D) into page
E) out of page

15. 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
Note if v is parallel to B then F=0 V q B

17. ACT: Moving Charges F = q v B sin(q) B
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?
A) 1 B) 2 C) 3 D) All Same
2) The force on particle 3 is in the same direction as the force on particle 1.
A) True B) False
F = q v B sin(q) B 3 2 1

18. Comparison Electric vs. Magnetic
Electric Magnetic
Source: Charges Moving Charges
Act on: Charges Moving Charges
Magnitude: F = Eq F = q v B sin(q)
Direction: Parallel to E Perpendicular to v,B

19. 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?
A) Into Page B) Out of page C) Left D) Right
FE would be up so FB must be down.

21. 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:
Work out in real time
Uniform B into page 41

22. Motion of Q in uniform B field
Force is perpendicular to B,v
Force is perpendicular to B,v
B does no work! (W=F d cos q )
Speed is constant (W=D K.E. )
Circular motion
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

23. 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

25. 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

26. ACT
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 48

27. extra questions to try B V 1 x x x x x x x 2
Which angle do you use to determine the magnitude of the force on the charged particle? (q1, q2 , either one).
Below I have drawn the trajectory for two charged particles traveling through a magnetic field. Is particle 1 positive or negatively charged?
If particles 1 and 2 have the same mass and velocity, which has the largest charge? V q1 q2 1
x x x x x x x 2

28. Summary We learned about magnetic fields B
We learned about magnetic forces on moving charged particles