unit 5: magnetism

Unit 5: MagnetismPhysics B

Bell WorkUsing 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 chargeParticle 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 WorkGiven this crystal of Cadmium (Cd) and

Titanium (Ti), determine if the structure isferro or anti-ferromagnetic at the

atomic and crystal levels.

Type 2: Electro-Magnets

Directions of fields…

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.

https://www.youtube.com/watch?v=1TKSfAkWWN0

PracticeI

I I

Making currents into looks

B = magnetic fieldµ = permeability of free spacen = number of turnsI = current

Geo-dynamo

Magnetic Field App & Gauge Bozons• http://www.falstad.com/vector3dm/

The Tesla!

https://www.youtube.com/watch?v=pPnGvjmIgZA

A charge traveling through a magnetic field experiences a force…

Magnetic Force [N] = Charge [C] * velocity of charge [m/s] * magnetic field strength [T] * sine of the angle between v & B

Quantifying Magnetism

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?

Bv

Q

l

Since there’s a force….We can analyze the Work done on the charge…

So, the work done on a charge is…

What are the units on this quantity?

EMF: Electromotive ForceThe force on charges due to a magnetic field, that act like a potential

difference, causing currents in wires.

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 Work1. 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…

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

2𝜋 𝑑

In the event of two wires, creating their own magnetic fields, and exposing each other to those fields….

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Two parallel wiresA. 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.

What are the units of magnetic flux?https://www.youtube.com/watch?v=NqdOyxJZj0U

https://www.youtube.com/watch?v=txmKr69jGBk

Magnetic Induction

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

http://sdsu-physics.org/NaturalScience100/Topics/3Mind/3magnetism_induction.html

Recap