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Unit 5: Magnetism
Physics B
Bell Work
Using the items at your desk, make as many observations as possible
about the properties of magnets.
Shape of the Magnetic Field
http://phet.colorado.edu/en/simulation/magnets-and-electromagnets
Properties of Magnets
• If you break a magnet into pieces what happens?
• If this happens, what does this mean about ‘monopoles’?
But, what causes magnetism?
Type 1: Permanent Magnets
1. Properties of Particles with charge
Particle
Mass
Charge
Proton
Electron
Does this mean all matter is magnetic?
2. The Atom
Ferromagnetic / Anti-ferromagnetic
www.ptable.com
3. Crystals
4. Domains
How do we know the moon use to have a magnetic field?
Bell Work
Given this crystal of Cadmium (Cd) and
Titanium (Ti), determine if the structure is
ferro or anti-ferromagnetic at the
atomic and crystal levels.
Type 2: Electro-Magnets
Directions of fields…
https://www.youtube.com/watch?v=1TKSfAkWWN0
Currents create magnetic fields!
• Right-hand-rule: Put your thumb in the direction of the current, and
your fingers wrap around in the direction of the magnetic field.
Practice
I
I
I
Making currents into looks
B = magnetic field
µ = permeability of
free space
n = number of turns
I = current
Geo-dynamo
Magnetic Field App & Gauge Bozons
• http://www.falstad.com/vector3dm/
The Tesla!
𝑉𝑠
1𝑇 = 2
𝑚
https://www.youtube.com/watch?v=pPnGvjmIgZA
Quantifying Magnetism
A charge traveling through a magnetic field experiences a force…
𝐹𝐵 = 𝑞𝑣𝐵sin(𝜃)
Magnetic Force [N] = Charge [C] * velocity of charge [m/s] * magnetic
field strength [T] * sine of the angle between v & B
New Right-hand Rule…
Practice!
• A proton moving east experiences a force of 8.8 x 1019 N upward due
to the Earth’s magnetic field. At this location, the field has a
magnitude of 5.5 x 10-5 T to the north. Find the velocity of the
particle.
A charge moving through a magnetic field follows a circular path…
Another Thought:
• A wire, with a charge Q lays in a
magnetic field pointed out of the
page…
• If the wire is physically moved to the
left…
• The force is in the _______ direction.
• What does this mean?
Since there’s a force….
We can analyze the Work done on the charge…
𝑊 = 𝐹𝑑
𝑊 = 𝑄𝑣𝐵 ∗ 𝑙
So, the work done on a charge is…
𝑊
= 𝑣𝐵𝑙
𝑄
What are the units on this quantity?
EMF: Electromotive Force
The force on charges due to a magnetic field, that act like a potential
difference, causing currents in wires.
𝐸𝑀𝐹 = 𝑙𝑣𝐵
New Problem
• A wire, with 6 Ω of resistance, and 12 m of length, sits in a magnetic
field of 2 T in the same position as the previous example. Calculate
the magnitude of the current.
• Note: Recall Ohm’s Law
Bell Work
1. Given the setup to the left, does a
current flow through the wire?
2. Given the initial conditions:
- v = 5 m/s
- length = 5 m
- the work done on each
charge due the magnetic field is 9 V
Determine the strength of the
magnetic field.
Magnetic Force on a current carrying
conductor…
𝐹𝐵 = 𝐵𝐼𝑙
Magnetic Force [N] = Magnetic Field [T] *
Current [A] * length of conductor [m]
Practice
• A wire 36 m long carries a current of 22 A from east to west. If the
magnetic force on the wire due to Earth’s magnetic field is downward
(towards Earth) and has a magnitude of 0.04 N, find the magnitude
and direction of the magnetic field at this location.
Note…
𝜇0 𝐼
𝐵=
2𝜋𝑑
In the event of two wires, creating their own
magnetic fields, and exposing each other to those
fields….
Two parallel wires
A. Currents flowing in the same direction.
Will the wires attract or repel?
B. Currents flowing in the opposite direction.
Will the wires attract or repel?
Changing Magnetic Fields generate currents…
Currents can be created by changing magnetic fields, in an attempt to
RESIST the change.
The change occurs by changing the FLUX of the magnetic field through
a loop of a conducting material.
Φ = 𝐴𝐵𝑐𝑜𝑠(Ө)
What are the units of magnetic flux?
https://www.youtube.com/watch?v=txmKr69jGBk
https://www.youtube.com/watch?v=NqdOyxJZj0U
Magnetic Induction
𝑑Φ𝐵
𝐸𝑀𝐹 = −𝑁
𝑑𝑡
Electromotive Force = - # of loops x the change in the Magnetic Flux
over some time interval
So changes to flux occur when:
• The area the field goes through changes.
• An area is exposed to a magnetic field or magnetic field strength
changes.
• The angle the loops is in the magnetic field changes.
Example 1: Changing Area
Example 2: Magnetic Field Strength Changes
Example 3: Changing Angle
Applications of the concepts…
Transformers
Recap
http://sdsuphysics.org/NaturalScience
100/Topics/3Mind/3magne
tism_induction.html