1. Magnetism

2. ALL magnets have two poles
NORTH seeking pole
SOUTH seeking pole

3. Breaking a magnet produces two magnets!

4. Opposite poles attract and like poles repel

5. Magnetic materials

6. Iron (steel), Cobalt and Nickel
Magnetic materials
Iron (steel), Cobalt and Nickel

7. Magnetic induction

8. Magnetic induction
When a magnetic material is close to a magnet, it becomes a magnet itself
We say it has induced magnetism
magnet S N N S

9. Hard and Soft Magnetism

10. Soft Magnetism Pure iron is a soft magnetic material
It is easy to magnetise but loses its magnetism easily
before
after N S S N N S N
Not a magnet
Iron nail

11. Hard Magnetism Steel is a hard magnetic material
It is harder to magnetise, but keeps its magnetism (it is used to make magnets!)
before
after S N S N N S N N
It’s a magnet! S
Steel paper clip

12. Magnetic Fields

13. Magnetism
Since ancient times, certain materials, called magnets, have been known to have the property of attracting tiny pieces of metal. This attractive property is called magnetism. N S
Bar Magnet N S

14. Magnetic Poles S N
Iron filings
The strength of a magnet is concentrated at the ends, called north and south “poles” of the magnet. N S E W
Compass
Bar magnet
A suspended magnet: N-seeking end and S-seeking end are N and S poles.

15. Magnetic Attraction-Repulsion
Magnetic Forces: Like Poles Repel
Unlike Poles Attract

16. Magnetic Field Lines
We can describe magnetic field lines by imagining a tiny compass placed at nearby points. N S
The direction of the magnetic field B at any point is the same as the direction indicated by this compass.
Field B is strong where lines are dense and weak where lines are sparse.

17. Field Lines Between Magnets
Unlike poles N S
Attraction
Leave N and enter S N
Repulsion
Like poles

18. The Density of Field LinesDN
Line density DA
Electric field Df
Line density DA
Magnetic field flux lines f N S
Magnetic Field B is sometimes called the flux density in Webers per square meter (Wb/m2).

19. Origin of Magnetic Fields
Recall that the strength of an electric field E was defined as the electric force per unit charge.
Since no isolated magnetic pole has ever been found, we can’t define the magnetic field B in terms of the magnetic force per unit north pole. + E
We will see instead that magnetic fields result from charges in motion—not from stationary charge or poles. This fact will be covered later. +
B v v ^

20. Magnetic Force on Moving Charge
Imagine a tube that projects charge +q with velocity v into perpendicular B field. N S B v F
Experiment shows:
Upward magnetic force F on charge moving in B field.
Each of the following results in a greater magnetic force F: an increase in velocity v, an increase in charge q, and a larger magnetic field B.

21. Direction of Magnetic Force
The right hand rule:
With a flat right hand, point thumb in direction of velocity v, fingers in direction of B field. The flat hand pushes in the direction of force F. B v F B v F N S
The force is greatest when the velocity v is perpendicular to the B field. The deflection decreases to zero for parallel motion.

22. Direction of the magnetic force? Right Hand Rule
To determine the DIRECTION of the force on a POSITIVE charge we use a special technique that helps us understand the 3D/perpendicular nature of magnetic fields.
Basically you hold your right hand flat with your thumb perpendicular to the rest of your fingers
The Fingers = Direction B-Field
The Thumb = Direction of velocity
The Palm = Direction of the Force
For NEGATIVE charges use left hand!

23. Example B = -x v = +y F = +z B = -z B =+Z v = +y v = +x F = F = -x -y
Determine the direction of the unknown variable for a proton moving in the field using the coordinate axis given +y
B = -x
v = +y
F = +z +z +x
B = -z
v = +y
F =
B =+Z
v = +x
F = -x -y

24. Example
Determine the direction of the unknown variable for an electron using the coordinate axis given. +y
B = +x
v = +y
F = +z +x +z F B
B =
v = - x
F = +y -z
B = +z
v =
F = +y +x

25. S N S N S N Force and Angle of Path
Deflection force greatest when path perpendicular to field. Least at parallel. S N S N B v F
v sin q q S N

26. Example 1. A 2-nC charge is projected with velocity 5 x 104 m/s at an angle of 300 with a 3 mT magnetic field as shown. What are the magnitude and direction of the resulting force?
Draw a rough sketch.
v sin f v
300 B v F B
q = 2 x 10-9 C v = 5 x 104 m/s B = 3 x 10-3 T q = 300
Using right-hand rule, the force is seen to be upward.
Resultant Magnetic Force: F = 1.50 x 10-7 N, upward

27. Forces on Negative Charges
Forces on negative charges are opposite to those on positive charges. The force on the negative charge requires a left-hand rule to show downward force F. N S N S F B v
Left-hand rule for negative q B v F
Right-hand rule for positive q

28. Indicating Direction of B-fields
One way of indicating the directions of fields perpen-dicular to a plane is to use crosses X and dots · :
A field directed into the paper is denoted by a cross “X” like the tail feathers of an arrow.
X X X X X X X X X X X X X X X X
· · · ·
A field directed out of the paper is denoted by a dot “ ” like the front tip end of an arrow.

29. Practice With Directions:
What is the direction of the force F on the charge in each of the examples described below? Up F + v
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 v +
Left F
· · · · Up F
· · · · - v - v F
Right
negative q

30. Zero deflection when FB = FE
Crossed E and B Fields
The motion of charged particles, such as electrons, can be controlled by combined electric and magnetic fields.
Note: FE on electron is upward and opposite E-field.
x x x x x x x x + - e- v
But, FB on electron is down (left-hand rule). B v FB - B v FE E e- -
Zero deflection when FB = FE