Download 1-Electromagnetic Forces - MrD-Home

What we’ll need for today…
•
•
•
•
•
Magnets (bar and horseshoe)
Iron filings
Compasses
Two wires, 4 batteries in series, light bulb
Electromagnets (solenoids)
James Clerk Maxwell
Michael Faraday
Electromagnetism
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?
Electromagnets
Arranging 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 RHR
For 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 RHR
For 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