1. Unit 9: Electromagnetism
Lesson 1: Magnetic Fields

2. Magnetism
A force in nature that attracts certain metals, such as iron, nickel & cobalt.
Magnets are surrounded by magnetic fields (regions where magnetic forces act).
Magnetic field lines (show direction of force):
10 min
- Point the way a compass would (from magnetic North to South)
Density = field strength
(strongest at poles in a bar magnet)

3. What causes magnetism?
- In normal materials, the spins are randomly oriented.
Electrons spin:
- In magnetic materials (can be affected by magnetic forces) there is some interaction between spins.
10 min
- In magnetic materials, the spins tend to line up in small areas called “domains”:
- In a magnet (produces magnetic forces), the domains are lined up.

4. The Magnetic Field Around a Wire
- All wires carrying current produce a magnetic field, B
- Direction: determined by the right hand rule (RHR)
10 min
Example: I
(I out of page) B

5. Brain Break!

6. Magnetic Field in a Solenoid
To determine direction:
consider one loop of wire B
Solenoid: a long straight coil of wire used to generate a magnetic field. I
10 min
Whole coil:
(similar to a bar magnet)
Note: B direction inside coil
new RHR
thumb = magnetic field
fingers = current

7. Example 2: Which end of the battery is positive?
Which end is North? N
Example 2: Which end of the battery is positive?
10 min
Left side of battery

8. Homework:
Pg. 206 #1 and 2

9. Unit 9: Electromagnetism
Lesson 2: Magnetic Fields and Forces

10. Magnetic Field in a Solenoid (magnitude)
Example: Find the magnetic field in the center of a solenoid of diameter 25 cm and length 32 cm with 500 turns and a current of 250 mA.
10 min

11. Quantitative Measure of Magnetic Forces
Magnetic fields cause forces on moving charged particles. →
B = magnetic field →
v = velocity of particle
q = charge
10 min →
F = force on the charge
θ is the angle between the velocity and the magnetic field
Note: when the angle is 90°, sinθ = 1 F
|F|= qvBsinθ
B =
qvsinθ
Direction – use right hand rule
Newton
B is measured in Tesla =
Coulomb•m/s

12. Right Hand Rule F Method: Thumb = force Pointer finger = velocity
Positive charge
5 min
Method:
Thumb = force
Pointer finger = velocity
Middle finger = magnetic field
Note: negative charge - reverse F

13. E.g. Positive charge (coming out of the page):
E.g. What direction is the force? B B
(out of page) v v -
10 min
What’s the direction of the magnetic force?
Force = up (negative charge)
Force:
left

14. Brain Break!

15. Application: Velocity Selector
E.g. Protons enter a region of uniform electric field 500 N/C as shown. v
10 min
Find the magnitude and direction of a magnetic field that would allow protons with velocity 2.0 x 106 m/s to pass through in a straight line.
Direction :
into page
FB = FE } FB
qvB = qE
balanced E
500
B = =
= 2.5 x 10-4 T v
2x106 FE

16. Homework:
Pg. 216 #1-3
Pg. 217 #1-4
10 min

17. Unit 9: Electromagnetism
Lesson 3: Moving Charges in Magnetic Fields

18. Force on a Current in a Magnetic FieldB
wire θ
θ = 90o
F = IlB q I
F = IlBsinθ l
Example: A wire carrying 20 A runs East to West along the equator where B = 5.0x10-5 T. Find the force per meter on the wire (include direction).
10 min N B
F = IlB E W I
= (20)(1)(5x10-5) S
= 1.0 x 10-3 N
(toward the ground)

19. Motion of Charged Particles in Magnetic Fields
What happens when a proton enters the Earth’s magnetic field (5.0x10-5 T), in a direction perpendicular to the field lines, at a velocity of 3.0 x 106 m/s?
x x x x x
F is always perpendicular to v
x x x x x F
Work = F||•d
= 0!
x x x x x F
Therefore, |v| stays constant.
x x x x x v
15 min
Uniform circular motion!
x x x x x
Find the radius of the circle:
F = ma
qvB = mv2/R mv
(1.67x10-27)(3x106)
R = =
= 630 m qB
(1.6x10-19)(5x10-5)

20. Brain Break!

21. Find the velocity of the helium nuclei.
Try This: Helium nuclei enter a region of uniform magnetic field of 3.0 x 10-3 T (perpendicular to field) and make a circular arc of radius 3.7 m.
Find the velocity of the helium nuclei.
m = 4(1.67x10-27 kg) q
= 2e
= 3.2x10-19 C
F = ma
mv2
qvB =
15 min R
qRB
(3.2x10-19)(3.7)(3x10-3)
v = = m
6.67x10-27
v =
5.3 x 105 m/s

22. Cathode Ray Tube (CRT)
Example: Electrons accelerated through volts in a CRT enter a region of uniform perpendicular magnetic field and follow a curved path of radius 37 cm. Find the magnetic field strength.
KE gained = PE lost
15 min
Homework:
Pg. 223 #1a, 3-5
= T