1. @ the end of the powerpoint
Magnetism-1
The Final Frontier
You will be Turning in
the Assignment
@ the end of the powerpoint

2. 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 N S
Bar Magnet

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

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

5. Magnetism is induced by aligning areas called domains within a magnetic field.

6. Magnetism originates in the motion of the electrons in iron atom.
Spinning electrons act like tiny magnets.
Cancellation of this effect occurs in most materials.
 Iron, nickel, cobalt are exceptions.

7. Magnetic Field Lines, B
The direction of the magnetic field, B, at any point is the same as the direction indicated by this compass. N S
Field, B is strong where lines are dense and weak where lines are sparse.
Lines are Out of North and INTO South
B, Field lines, which are always closed loops.

8. Magnetic Field Lines N S N Unlike poles Attraction Leave N and enter S
Repulsion

9. Compasses
Compasses
Compasses are magnets that can easily rotate and align themselves
If they are allowed to select their own orientation, magnets align so that the north pole points in the direction of the magnetic field

10. A compass points to the Earth’s North geographic pole (which is near the south magnetic pole) Is the North Magnetic Pole the north pole of the Earth’s Magnetic field?

11. Magnetic “Monopoles” Do not exist!
In this way, magnetic poles differ from electric poles (charges), which can be separated into electric monopoles.
Electric monopoles exist as either a negatively charged or a positively charged object.

12. Origin of Magnetic Fields
Recall that the strength of an electric field E was defined as the electric force per unit charge. (Yea old positive test 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.. +
B v v ^

13. Magnetic Force on Moving Charge
Imagine a tube that projects charge +q with velocity v into perpendicular B field. N S B v F
Experiments show:
F = qvB
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.

14. Definition of B-field Experimental observations show the following:
Just like in work problems… the angle between the force and the directions of q’s velocity are important!
Experimental observations show the following:
By choosing appropriate units for the constant of proportionality, we can now define the B-field as:
Magnetic Field Intensity B:
A magnetic field intensity of one tesla (T) exists in a region of space where a charge of one coulomb (C) moving at 1 m/s perpendicular to the B-field will experience a force of one newton (N).

15. Units of Magnetic Field
Tesla (SI)
N/(C m/s)
N/(A m)
Gauss
1 Tesla = 104 gauss

16. 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.

17. Direction of Magnetic Force- method 2

18. 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. F B v
Left-hand rule for negative q N S B v F
Right-hand rule for positive q N S

19. Indicating Direction of B-fields
One way of indicating the directions of fields perpendicular 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.

20. 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

21. Magnetic Force on Particles
Magnetic fields cause the existence of magnetic forces.
A magnetic force is exerted on a particle within a magnetic field only if
the particle has a charge.
the charged particle is moving with at least a portion of its velocity perpendicular to the magnetic field.

22. Magnetic Force on a Charged Particle
magnitude: F = qvBsin
q: charge in Coulombs
v: speed in meters/second
B: magnetic field in Tesla
: angle between v and B
direction: Right Hand Rule
FB = q v x B (This is a “vector cross product”)
two examples…

23. 1. The proton is released from rest at point P in a magnetic field B having intensity 10‑l T and directed into the page as shown below.
FB =𝓺𝒗𝓑sin θ = 0
The proton is at rest (𝒗=0) & is not crossing any of the magnetic field lines so no from magnetic field on the proton.
2. In the same magnetic field, the proton at point P has velocity v = 105 meters per second directed to the right as shown below.
FB =𝓺𝒗𝓑sin θ
= (1.6x10-19C)(105m/s)(10-1T)sin 90
=1.6 x N
Using the RHR the magnetic field exerts a force(initially upward) that will cause the proton to travel in a circular path.

24. Review problem with an ELECTRIC FIELD
So the particle does NOT need to be moving in the electric field
Calculate the force and describe the path of this electron.
FE = Eq
ForceElectric Field = Electric Field x charge
E = 2000 N/C
FE = (2000 N/C)(1.6 x 10-19C) FE
FE = 3.2 x 10-16N e-
300,000 m/s
and the path?
I know that electric field lines come out of positive, so the top of the slide is positive so the force on the electron is upward. So the path is…

25. B = 2000 mT Sample with magnetism e- 300,000 m/s F = qvB
Calculate the force and describe the path of this electron.
F = qvB e-
300,000 m/s
B = 2000 mT

26. Magnetic forces…
are always orthogonal (at right angles) to the plane established by the velocity and magnetic field vectors.
can accelerate charged particles by changing their direction.
can cause charged particles to move in circular or helical paths.

27. Magnetic forces cannot...
change the speed or kinetic energy of charged particles.
do work on charged particles.

28. Magnetic Forces are Centripetal
Please turn in your paper.
SF = ma
FB = mac
qvBsin = mv2/r
qB = mv/r
q/m = v/(rB) V F V F V F V F B