Transcript Lesson 2: Magnetism

Question of the Day:
Sketch the magnetic (B) field around a bar magnet.
C
F
A
S
N
D
B
E
Which way will the north pole of a compass point
if placed at point A-F?
What is the direction of the B-field at A-F?
A compass works just like iron filings:
What do these have in common?
north and south pole
II. All magnets have a ___________________________.
S
N
If you break a magnet…
S
N
will become an entire magnet
…each piece ________________________________:
N
S
N
S
again have
If you break it again, each piece will _____________
both a N- and S-pole A separate, single, N or S
_____________________.
magnetic pole (called a ________________
) has
monopole
never been observed
_____________________________
.
Earth has a magnetic field that is __________________
similar to
the field of ______________________.
a bar magnet
magnetic
______________
axis
true axis
________
bar
_________
magnet
_________
S
N
Earth
_________
Earth’s B field:
--protects the planet from ______________________
cosmic rays
borealis (northern) lights
--produces the _________________________________
--at the surface is a result of the magnetic effects of:
rotating currents in the outer core (90%)
a/ __________________________________________
magnetic rocks in the crust
b/ __________________________________________
electric currents in the ionosphere
c/ __________________________________________
ocean currents
d/ ___________________________________________
The magnetosphere—a magnetic “cocoon”:
The Origin of the __________________________
:
aurora borealis
Charged particles from the Sun
experience a ________________
magnetic
perpendicular
force that is _________________
velocity
to their ______________
and to the
magnetic field. This is a
____________
centripetal
________________
force Fc that
spiral
causes them to _____________
around the B field. As they
spiral down to Earth, they knock
electrons out of atmospheric
___________
atoms . When these electrons
________
re-combine
_______________
with their atoms,
emit light of different colors.
they ___________________________
path of a
charged
_________
particle
Fc due
____
to B field
B_______
field
line
Earth's field makes an angle to its
inclination
surface called the _________________________.
What is the
angle at the
poles?
900
What is the
angle at the
equator?
00
How to measure
the inclination of
Earth's B?
magnetized
steel needle
18 cm
Compass and dip needles were
used to find magnetite in
Sweden in the Middle Ages,
making the magnetic method the
oldest of all applied geophysical
techniques. This method of
magnetic prospecting for ores
was used extensively for iron
mining up until the early
decades of the twentieth
century. However, the dip circle
is not a sensitive enough
instrument for most ore
prospecting purposes and has
been replaced with groundbased versions of
magnetometers used in
aeromagnetic surveys.
The N- and S-poles of Earth’s B field:
spin
a/ are not the same as the __________
(true) poles
slowly wander around
b/ _____________________________
reverse
c/ Sometimes ____________
polarity so that
N becomes S
____________________
and vice versa. The last
750,000
time this occurred was about ____________
y ago
Magnetic ___________________
is the angle that the
declination
true north.
_______________
north makes with _________
magnetic
130
In Ilion, the declination
is about ___________
130 W
magnetic
__________
north
true
________
north
magnetic N pole
The wandering _______________________
130
New York
_______________
Magnetic declination changes…
…in space:
…and in time.
Neutron stars are large stars that have collapsed
until they are planet sized. As they collapse, their
magnetic fields intensify. As they rapidly rotate, their
magnetic field sweeps around like a lighthouse.
They were originally called pulsars.
The Sun’s magnetic field…
1. produces sunspots
2. reverses itself every 11 y
(next peak: 2012….)
3. affects climate on Earth.
magnetograms of the sun: dark is one pole,
light is the other
In 2012:
1. A comet may strike Earth.
2. Yellowstone Caldera may erupt.
3. The Mayan Calendar predicts the End of the World.
4. The magnetic field of Earth may reverse
5. The sunspot cycle will reach its peak.
Which one of these facts is an inference most likely
to come true?
Magnet therapy:
Oersted’s
__________________experiment
(1819 in Copenhagen):
wire
S
N
compass
battery
switch
deflected the compass
An electric current ___________________________.
 Currents, I, _____________________________.
cause magnetism, B
Electricity
________________
and Magnetism are closely related.
Each one can induce (give rise to) the other. This is
electromagnetic
the basis for the ______________________
generation
of power .
I. The Magnetic Fields of Current-Carrying wires:
wire vertically. Support a piece
Experiment: Hang a _______
current on
of paper horizontally. Turn _____________.
Sprinkle iron
B field
filings to see the ______________
pattern.
nice
animation
right hand rule
The _____________
tells you which one of
the two possible ways
that the B field points.
The B field of a long straight wire has the shape of
concentric circles
_________________________
.
View directly at
wire with I towards
I
you:
B
B
B
side view:
>
I
coil
Ex: Twist a wire into
coil
the shape of a _______.
I
The B field of the
coil is shaped like
bar magnet
a _______________:
Side view:
N
N
S
S
Electromagnets
_______________________
are magnets made with coils.
solenoids
They are also called ____________________
.
To strengthen its B field, use:
more current
1. _______________________
more coils
2. ______________________
a ferromagnetic core
3. __________________________
Fe
C
o
r
e
What are electromagnets used for?
Plus electric motors
and generators…
The B field of the electromagnet
turns its iron core into a magnet.
The core will then be attracted
into the coil's field.
 This is how solenoids work.
Analog ammeters (and voltmeters) use coils, too:
•Current is used in coils to create electromagnets.
•The electromagnets repel/attract and move the needle.
•The stronger the current, the more the force, and
the more the needle moves.
II. Magnetism in Solids.
currents
Magnetism in solids is caused by the ______________
electrons
that result from the motion of ________________
as they
"spin" on their axis
1/ _____________________________
, or
"orbit" their nuclei
2/ _____________________________
within each atom.
An atom:
e- orbit
Think of a
spinning
electron:
nucleus
as a tiny
bar
_______
magnet
___________:
e- spins
on axis
Isn't
it
amazing
that
Earth's
magnetic
field
is
similar
to
that
of
an
electron!!!!
Types of magnetism:
1. ferromagnetism:
strong
--___________________
attraction
Fe, Ni and Co
--only in ______________________
unpaired
--caused by ___________________electrons
that
act _______________________
cooperatively
Why ferromagnetism only occurs in certain atoms:
positions
Electrons can only "fit" in certain _______________
around
2
orbitals
the nucleus called _______________
. At most, only ___
electrons, spinning in ________________
directions, can fit
opposite
orbital
in each _______________
.
orbital
= represents an electron ______________
in an atom
one way
= represents an e- spinning ______________
the other way
= represents an e- spinning ______________
unique
Each element has its own ______________
arrangement
number
of orbitals and ______________
of electrons. Whether
an atom is ferromagnetic or not depends on whether
filled up
the ________________
are _______________
or not.
orbitals
not magnetic
cancel  _____________
filled orbital  spins ________
= ________
unfilled orbital 
= _________

ferromagnetic because
__________________________
spins act like magnets
__________________________
2. diamagnetism:
repulsion
--weak magnetic ________________
--occurs in water, glass, Cu, Pb, salt, wood,
most gases, plastics, etc.
3. paramagnetism:
attraction
--weak magnetic _________________
--Al, O, Na
In a ferromagnetic solid (iron, nickel or cobalt), atoms
with unfilled electron orbitals organize themselves into
domains -- area in which the electrons are lined up
Ex. An unmagnetized ferromagnetic material has
random domains:
domains
= spin of
an electron
Ex. A magnetized ferromagnetic material has domains
that are aligned with each other
external magnetic
field
4. Human
magnetism:
Franz Mesmer (mesmerized…)
Ex. Credit cards have a magnetic strip that records
information:
Don’t do this!
What kind of field surrounds …
1. a neutron at rest?
2. a proton or an
electron at rest?
3. a moving* proton
or electron?
Field
caused
by:
gravitational g
mass m
gravitational g
electric E
gravitational g
mass m
charge q
electric E
mass m
charge q
magnetic B
current I
"relative"
*The motion is _________________
. This means that
at rest
you will also measure a B if the charge is __________
past it
and you _________________
.
Uses of strong magnetic fields:
1. _______________________________________
medical - NMRs
power transmission
2. ______________________________________
transportation – maglev trains
3. _______________________________________
computer memory
4. ______________________________________
research – contain fusion reactions
5. _____________________________________
To produce strong magnetic fields,
high currents are needed. But this causes
___________________
I2R
joule
______________
or ___________
heating. To reduce
superconductors
the heating, ____________________________
are used
resistance
because they have no __________________________.