Magnetism in materials
l What causes magnetism
in materials, like this bar
magnet?
l Can imagine it has
something to do with
current loops
l Each electron inside an
atom circles the atom in
about 1.6X10-3 s
u
u
so it creates a magnetic
field of 20 T at the center
of the atom
if every electron did this,
then very strong magnetic
fields would be created
inside each atom and
inside every material
Rutherford model
Copyright (c) Grolier Interactive Inc.
Not so fast
Rutherford model
l Magnetic field created by one
electron travelling in one
direction is most often
cancelled out by an electron
travelling in the other direction
l Magnetic properties of most
materials explained by the fact
that the electron is not only
moving in a circle while
orbiting the nucleus, but is
also spinning on its axis
l This spin is sort of a current
loop as well
l But for most types of atoms,
electrons usually pair up with
spins opposite each other
l So again, a cancellation of
magnetic fields
Copyright (c) Grolier Interactive Inc.
Ferromagnetism
l In some materials, such
domains are from 10-4 cm to
as iron, cobalt, and
0.1 cm in size
nickel, the magnetic
fields produced by
electron spins do not
cancel completely
l These materials are
called ferromagnetic
l Groups of nearby atoms
tend to have spins
aligned in same direction The material does not produce a
noticeable magnetic field because
u domains
the domains are all pointing in
random directions
Fig. 19.37a, p.609
Applying an external magnetic field
l Suppose I apply an
external magnetic field to
my ferromagnetic
material
l Domains aligned with
respect to external field
tend to grow at the
expense of those not
l i.e. the material becomes
magnetized
l In “hard” magnetic
materials, the
magnetization persists
even after the external
but I can change the direction of
field is removed
magnetization if I try hard enough
u a permanent magnet
Iron
l In “soft” magnetic
materials such as iron,
the domains tend to
randomize after the
external field is removed
because of thermal
agitation (or by banging
with a hammer)
l There are ways of
causing a lot of thermal
agitation and I can cause
even a permanent
magnet to lose its
magnetization
Diamagnetism
l Besides ferromagnetism,
there are two other types
of magnetism
u
u
paramagnetism (forget
about it)
diamagnetism
l Superconductors are
diamagnetic
u
u
this means they really,
really hate magnetic fields
…and will do anything they
can to get rid of them
Magnetic induction
l This next chapter is
mostly about Michael
Faraday
u
1791-1867
l Little formal
education; almost
entire self-taught
u
he was apprenticed
as a book-binder and
ended up reading
most of the books he
was supposed to bind
Fig. 20.p621, p.621
Royal Lectures
l He attended all of the
public lectures given by
the Royal Academy of
Sciencies
l Kept a notebook from the
lectures of Sir Humprey
Davies, bound it an
presented it to him
u
Davies made him a lab
assistant
l We’ve already
encountered a unit
named after him (the
Farad) and some of his
most useful ideas
u
electric and magnetic field
lines
because of his lack of formal
mathematical training, most of
Faraday’s thinking was intuitive
An important experiment
l This is his most famous
experiment
u
and he thought of it
while…
this is the experiment that
he set up
sitting in his laboratory
l In the early 1800’s he
was where we are now in
this course
u
u
strong electric fields create
magnetic fields (by
creating currents)
from symmetry it seemed
that strong magnetic fields
should be able to create
electric fields (and
currents)
what did he find?
let’s try our version.
Fig. 20.1, p.621
What he found
l A strong magnetic field
does not create an
electric current
l But we did notice a
current in the meter
when we first closed the
switch and just after we
opened it again
l So it’s not a magnetic
field that creates an
electric current; it’s a
changing magnetic field
Magnetic flux
…and it’s not the changing magnetic field per se but the changing
magnetic flux that creates the current
define the magnetic flux as
FB = BTA = BAcosq
think of it
as counting
the # of
field lines
passing thru
a surface
Magnetic flux
Fig. 20.3, p.622
Faraday’s law of induction
l The emf e equals the
time rate of change
of the magnetic flux
u e = - N DFB/Dt
this - sign is so important we’re
going to give it a name all to
itself: Lenz’s law
l We saw one way of
changing the
magnetic flux
u
here’s another
Fig. 20.4, p.623
Notice the direction of the current flow
l That’s the - sign
u
u
the direction of the
induced emf is such
that it tries to produce
a current whose
magnetic field
opposes the change
in flux thru the loop
Lenz’s law (the “Idaho
Republican law”)
s
any change is resisted
no matter what the
direction
Notice the direction of the current flow
I push the magnet
in; the coil is
pushing back
I pull the magnet
out; the coil is
pulling back
Lenz’s law
l Suppose I have a
conducting bar sliding on
conducting rails, in a
magnetic field pointing
into the page
l If it slides to the right,
what is the direction of
the current
u
what is the direction of the
force?
l What if the bar were
sliding to the left?
Quiz
1.
What is the magnitude of the
force on the proton?
a)
b)
c)
d)
e)
2.
1.6 X 10-19 N
5 X 105 N
15.0 N
4.4 X 10-4 N
.031 N
What is the direction of the
force on the proton?
a) to the right
b) to the left
c) up towards the top of the
page
d) down towards the bottom of
the page
e) out of the plane of the page
.1 m
I
v
A current of 1 A (downward)
creates a magnetic field at the
position marked by a smiley face.
A proton at that position
(charge =1.6 X 10-19 C) is
travelling with v = 5 X 105 m/s
in the direction indicated.