Download 6. Magnetism

Magnetism
Force of Mystery
Presentation Text ©2001 Philip M. Dauber
as modified by R. McDermott
Magnetic Poles
North and South
Like poles repel N-N S-S
Unlike poles attract N-S
Poles are named for which way they point if
suspended (so they rotate freely)
A magnetic north points to geographic north
Magnetic Poles Are Not Charges
A north pole is not a positive charge.
A south pole is not a negative charge.
Magnets do not exert forces on stationary charges.
Charged objects are not affected by magnets.
Magnetic Poles Are …. Poles
Single poles cannot be isolated
Magnetic monopoles do not exist in nature
Break a magnet:
N
S
Get two smaller ones:
N
S N
S
Ferromagnetic Materials
Show strong magnetic effects
Iron
Cobalt
Gadolinium
Neodymium
Ferromagnetism
Ferromagnetic material contains “domains”
1 mm in length and normally random in direction
Each acts like tiny magnet
Generally, domains cancel – no magnetic effects
An external field aligns domains (non-random)
A strong magnetic field can make other
ferromagnetic materials into permanent magnets
Electrons and Magnetism
Magnetism is electrical in origin
Magnetic fields are produced when charges
move
Even permanent magnets owe their strength to
electron “currents”
There is no way to “divide” a current to get N or
S pole
Stationary charges are unaffected by magnetic
fields, and do not generate magnetic fields.
Permanent Magnets
Hi tech
Neodymium
iron boron
magnets
Units of Magnetic Field
Tesla (SI Unit)
1 Tesla = 1 Weber/m2
Earth’s magnetic field = 5x10-5 Tesla
Earth’s Magnetic Field
Very weak
Like a bar magnet
North magnetic pole is at
the south geographic pole
South magnetic pole is at
the north geographic pole
Direction of Magnetic Field
The direction the north pole of a compass would
point when placed at that location
Demo
Magnetic Field
Magnetic field lines (flux) are measured in
webers
Magnetic lines of flux go from North to South
Electric Currents = Magnetism
Magnetic field around long straight wire
I
Demo
Right hand rules
determine the direction
of magnetic fields
–Point thumb in direction of current
–Fingers wrapped around wire point in
direction of magnetic field
Electric Currents = Magnetism
Magnetic field due to circular loops (solenoid)
Demo
– Grab loop with thumb of right hand in the
direction of conventional current.
– Fingers point in direction of field at center of
loop.
Right Hand Rule(s)
Long Straight Wire
– Point right thumb in direction of conventional current
– Fingers wrapped around wire point in direction of
magnetic field
Circular loop(s) of Wire
– Grab loop with thumb in current direction
– Fingers point in direction of field at center of loop
Force on a Charged particle in a
Magnetic Field
F = qvB sinQ
Magnetic force is perpendicular to both particle
direction and field (vector cross-product)
Demo
Charged Particle Path in Uniform
Magnetic Field
Circle or helix
F = ma
qvB = mv2/r (centripetal acceleration)
r = mv/qB (mass spectrograph)
Direction follows third right hand rule
Demo
Force on Current-Carrying Wire
F = BIL sinQ
Q is angle between
field and wire
I
Force is perpendicular to both
current and field directions
How can F = BILsinQ be Used
to measure a Field?
Hint: use a rectangular loop of wire
Demo
Magnetic Field Due to Straight Wire
B = m0I/(2pr)
-7
*m0 = 4px10 T-m/A
* Permeability of free space
I
Force Between Parallel Wires
F/ L = moI1I2/(2pd)
Force per unit length of wire
d is distance between wires
Unidirectional currents attract
Bidirectional currents repel
Induced EMF
 = LvB sinQ
Moving a wire across magnetic field lines
produces an EMF (potential difference) across the
wire.
This potential difference increases with increasing
wire speed.
The maximum EMF occurs when the wire is
moved perpendicular to the field lines.