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Magnetism
Force of Mystery
demo
Magnetism Standards
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Students know magnetic materials and electric
currents (moving electric charges) are sources of
magnetic fields and are subject to forces arising
from the magnetic fields of other sources. (Ch 36)
Students know how to determine the direction of a
magnetic field produced by a current flowing in a
straight wire or in a coil. (Ch 36)
Students know changing magnetic fields produce
electric fields, thereby inducing currents in nearby
conductors. (Ch 37)
Famous 19th Century Quote
“The nation that controls magnetism
controls the Earth”
Magnetic Poles
North and South
Like poles repel N-N
S-S
Unlike poles attract
N-S
Magnetic Poles Are Not
Charges
Single poles cannot be isolated
Magnetic Monopoles do not exist in
nature
N
Break a magnet:
Get two smaller ones
N
S N
S
S
Ferromagnetic Materials
Show strong magnetic effects
Iron
Cobalt
Nickel
Gadolinium
Neodymium
Permanent Magnets
Hi tech
Neodymium
iron boron
magnets
Magnetic Field
Earth has field
Lines go from North to South
Units of Magnetic Field B
Tesla (SI Unit)
Gauss (cgs unit)
1 Tesla = 104 Gauss
Earth magnetic field about 0.5 gauss
Direction of Magnetic Field
The direction the north pole of a compass would
point when placed at that location
Ferromagnetism
Magnet made of domains
1 mm length
Each acts like tiny magnet
Normally domain cancel
External field aligns domains
Strong magnet can make other
ferromagnetic materials into permanent
magnets
Electrons Have Spin
Even permanent magnets owe strength
to “currents”
No way to divide a current and get N or
S pole
Magnetism is electrical in origin
Earth’s Magnetic Field
Very weak
Like bar magnet
North magnetic pole
South magnetic pole
Electric Currents Produce
Magnetism
Magnetic field around long straight wire
I
Right hand rule
determines
direction of
magnetic field
Right Hand Rule(s)
Long Straight Wire (Rule #1)
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Point thumb in direction of current
Fingers wrapped around wire point in
direction of magnetic field
Circular loop of Wire (Rule #2)
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Curl fingers around wire with tips in field
direction
Thumb points in direction of current
Alternate (preferred) version of
Second RHR
Put curled fingers in current direction
around loop or loops; thumb points in
field direction INSIDE loop or coil.
Force on Current Carrying
Wire
F = BIL sinQ
Qis angle between
I
field and wire
Force is perpendicular to both
current and field direction
q
Third Right Hand Rule
Long straight fingers in (positive) current
direction (or direction of moving charged
particle). Curled fingers in magnetic field
direction, thumb points in direction of force
on current carrying wire or positive charged
particle
If particle is negative, change answer
Force on Moving Charged Particle in
Uniform Magnetic Field
F = Bqvsinq
This force is perpendicular to the
magnetic field and particle velocity
vector
Charge Particle Path in
Uniform Magnetic Field
Circle or helix
F = ma
qvB = mv2/r (centripetal acceleration)
r = mv/qB
Direction follows right hand rule
How can F = BIL sinQ
be Used to measure a
Field?
Hint: use a rectangular loop of wire
Force on a Charged particle in
a Magnetic Field
Demo
F = qvB sinQ
Force perpendicular to both particle
direction and field
Magnetic Field Due to Straight
Wire
B = m0I/2pr
F = BIL
m0 permeability of free space
4 p x 10-7
I
Force Between Parallel Wires
F/l = (mo/2p) I1I2/L
Force per unit length of wire
L is distance between wires
Parallel currents attract
Antiparallel currents repel
Electrical vs. Magnetic Forces
Similarities
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Both involve attracting and repelling
Both decrease with distance
Differences
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Isolated poles do not exist
Only electrical forces can be produced by
stationary charges
Only moving charged particles experience
magnetic force
Only electrical forces can do work
 Magnetic forces on charged particles are perpendicular to
field direction but electrical forces are in or opposite to
electric field direction