What we’ll need for today…

• Magnets (bar and horseshoe)

• Iron filings

• Compasses

• Two wires, 4 batteries in series, light bulb

• Electromagnets (solenoids)

Electromagnetism

James Clerk Maxwell Michael Faraday

Magnets: What do you know?

Magnets – Key Points• Have poles (N and S) rather than + and – for charges• Like poles repel; Opposite poles attract• Produce a magnetic field: B

similar to gravitational field: g

and electric field: E• Magnetic Flux refers to the density of field lines

The Tabletop Explainer…

Magnetic Field (B)

• Vector quantity (arrows)

• Points in direction a compass would point

• Runs from North to South

• Allows for FM: Magnetic Forces (the reason a compass needle moves!)

Where does the electro come in?

• Current carrying wire….

Current carrying wire…

• A static distribution of charges produces an electric field

• Charges in motion (an electrical current) produce a magnetic field

1st RHR A moving electric charge produces a magnetic field

•Thumb: Direction of Current

•Fingers: Curl in direction of magnetic field

What happens then…..

If we have a whole bunch of current carrying wire wrapped tightly?

ElectromagnetsArranging wire in a coil and running a current through produces a magnetic field that looks a lot like a bar magnet

Solenoid (electromagnet)

The 2nd RHR:

Fingers: Direction of current through solenoid

Thumb: Points to north pole

Cross section:

Magnetic fields inside a solenoid

B = µo I n B: Magnetic Field Strenth (Tesla T)

µo : Permeability of free space =

4π x 10-7 T·m/A

I: Current (Amperes A)

n: Loops per meter = N/l

N: total loops l: length

Example

A hollow solenoid is 25 cm long and has 1000 loops. If the solenoid has a diameter of 4.0 cm and a current of 9.0 A what is the magnetic field in the solenoid?

3rd RHR

Applies to:

1.Charges moving in a magnetic field

2.A current carrying wire in a magnetic field

Cross Product

Cross product: Vector product of two vectors. Gives a new vector that is orthogonal (perpendicular) to both

3rd RHR

Direction:

Thumb: current/particle motion

Fingers: Magnetic Field direction

Force: Palm (positive); Knuckle (negative)

Mass spectrometer

3rd RHRFor a charge moving in a magnetic field, a magnetic force is applied to it.

FM = q v x B (cross product)

For us…

FM = qvBsinθ q: charge

v: velocity

B: Magnetic Field strength

θ: orientation

Example

A proton is fired into a magnetic field as follows:

Find/show:

a)It’s path

b)FM

c)Radius of it’s path

3rd RHRFor a current carrying conductor, the magnetic force is as follows:

FM = B I l sin θ

If the conductor is perpendicular to the magnetic field:

FM = BIl B: Magnetic Field strength (T)

I: Current (A)

l: length of conductor (m)

θ: orientation

3rd RHR

For a current carrying wire in a magnetic field, a magnetic force is applied to it.

FM = B I L sinθ B: Magnetic Field strength

I: current

L: Length of wire in magnetic field

θ: orientation