Download Chapter 24 – Magnetism

Magnetism (Chapter 24)
MAGNETIC Poles, Forces and Fields
DEMO magnet and paper clips (Comment on the weakness of the gravitational field)
Similar to the arguments I gave back when we introduced the causes of all electrical
phenomena, I will repeat this for the magnetic phenomena.
1. What is the cause of all magnetic phenomena? A moving electric charge.
However, magnetism was first believed to be cause by magnetic poles. There are
two types of magnetic poles: North and South magnetic poles.
2. Since there are two types of magnetic poles, there are two types of magnetic forces:
attractive or repulsive
DEMO domain model and two magnets
3. The magnetic force is transmitted through the magnetic field (defined as B-field).
4. Biot-Savart law (inverse square law): The farther two magnets get from each other,
the weaker the magnetic force between it is.
FB ∝
1
(distance)2
Units of measurement: Gauss or Tesla
The B-field direction is measured using a compass.
DEMO Compass and magnet
Earth’s Geomagnetic Field
The earth behaves magnetically almost as if a bar magnet were located near its center.
The axis of this fictitious bar magnet does not coincide with the earth’s rotational axis but
is currently tilted about 11o from the Earth’s rotational axis.
Lecture 8-1
What is the cause of the earth’s magnetic field?
However, unlike the field of a bar magnet, Earth's field changes over time because it is
generated by the motion of molten iron alloys in the Earth's outer core (the geodynamo).
A cutaway artwork of the earth illustrating the convection within its interior and the
pattern of its magnetic field.
•
•
The Earth has a solid center (inner core, blue), which is surrounded by a liquid layer
(the outer core, yellow) where molten iron circulates (red arrows). The outer core is
surrounded by the mantle (red). The core rotates faster than the bulk of the planet
which means that the inner core gains a full turn on the rest of the planet every 400
years.
The convective motion of molten iron in the outer core combined with the Earth's
rotation is what produces the dipole magnetic field (blue arrows). That is, the
magnetic field is thought to be generated by the spiral movement of molten iron in
the Earth's liquid outer core (yellow), which acts as an electromagnet.
There are two key points about the geomagnetic field:
1. The Magnetic North Pole wanders, fortunately slowly enough that the compass is
useful for navigation. At random intervals (averaging several hundred thousand
years) the Earth's field reverses (the north and south geomagnetic poles change
places with each other). These reversals leave a record in rocks that allow
Lecture 8-2
paleomagnetists to calculate past motions of continents and ocean floors as a result
of plate tectonics.
• The magnetic pole moves from one pole to the other slowly over the years. For
example, it current location is about 770 km northwest of its position in 1904.
There has been a 5% decrease in the earth’s magnetic field over the past 100
years. If this steady decrease would continue, in 2000 years the earth’s magnetic
field would be “zero.”
• The magnetic pole of the earth has been known to reverse directions. In fact,
there have been 20 reversals in 5 million years. Most recent reversal was
700,000 years ago, than, 870,000 years, then 950,000 years.
2. The region above the ionosphere, and extending several tens of thousands of
kilometers into space, is called the magnetosphere. This region protects the Earth
from cosmic rays that would strip away the upper atmosphere, including the ozone
layer that protects the earth from harmful ultraviolet radiation. More on this later on.
The earth’s magnetic field plays a role in the way honeybees (as well as pigeons)
navigate between their hive and the flowers they visit.
Magnetic Domains and Creating a B-field
A bar magnet has two poles. If one breaks a bar magnet to isolate the North and South
Pole, one will instead find that the bar magnet has now become two magnets. If I
continue to break the magnet into smaller and smaller pieces, eventually I will get to the
atomic level and find that the real reason why the bar magnet had a B-field was that the
electrons where circling the nucleus. Because the electrons can either rotate either
Counterclockwise (CCW) or Clockwise (CW), we can determine the direction of the
rotations in term of “spin” by using the Right-hand Rule. If my fingers/hand encircles the
motion of the electrons in the CCW direction, my thumb points upward. This thumb
direction represents the spinning motion of the electrons and is called spin-up.
However, if I repeat this process for the CW motion, I have to rotate my fingers/hand
upside-down so that my thumb now points downwards; this is called spin-down.
When there is a collection of atoms with the same spin orientation, they these atoms
form a magnetic domain (a little neighborhood of similar spinning atoms). Depending
on whether the magnetic domains are align or not, then a magnetic field is produced.
That is,
What is the difference between a strong (Neodymium) and weak (cow) magnet?
Lecture 8-3
MAGNETIC DOMAINS – Induced Magnetism
Analogy: electric polarization occurs with a charged balloon polarizing the atoms
in the wall (i.e., it causes the atoms to rearrange their charge distribution),
and results in the balloon sticking to the wall.
So how do the refrigerator magnets work? Before the magnet is placed on
the refrigerator, the domains are randomly arranged (unmagnetized). When
the magnet is brought to the refrigerator, the magnet induces magnetization:
similar to electric polarization, the magnetic domains rearrange themselves so that the
surface of the refrigerator is magnetized to be magnetically north. As a result, the
magnet sticks to the refrigerator.
Example: why does a magnet pick-up several paper clips?
DEMO magnet and paper clips
Applications of domains:
1. Magnetic strips on credit cards
DEMO domain model and magnet
2. MRI domains
Electric Currents and Magnetic Fields
If a moving charge produces a B-field, then electric current will also produce a B-field.
smaller current → smaller B-field produced
B-field ∝ current 
→
larger current → larger B-field produced
For a single current-carrying wire, the B-field encircles the wire according the Righthand Rule. The Right-hand Rule says that if your thumb points in the direction of
the current and let your fingers curl around the direction of the thumb, the B-field by
this wire encircles the wire in circular paths. If we repeat this process with the
solenoid, the B-field of the solenoid is very similar to a bar magnet as shown below.
Since the solenoid acts like a magnet, it can repel other magnets when similar pole
magnets are inset into its core.
more turns → stronger B-field
B-field of solenoid ∝ ( number of turns of wie )( current ) 
→
less turns → weaker B-field
DEMO Solenoid and compass on overhead; eject magnets; Domain model with coil
If you look at this magnetic launcher, note that it has a solenoid coil. So when I
turn on the voltage, a current will produce a B-field that is north pointing
upwards. Without going into the details, if I place an aluminum ring around the
solenoid, the solenoid will in turn produce a current in the aluminum ring such
that it produces its own magnetic field that is the same B-field of the solenoid. As
a result, these two fields interact and the ring is repelled upwards. See picture
on the right. Here is the important detail: if you want to make a stronger B-field, it
is common to insert inside the bore of the solenoid an iron core. So how does
this increase the B-field field strength of the solenoid? When the magnetic
Lecture 8-4
launcher is turned on, the B-field of the solenoid will magnetize the domains of the iron
core so that the B-field of the solenoid and the domains add together:
As a result, the combined B-field of the two are much stronger and the ring will not be
launcher up much higher as before.
DEMO Magnetic launcher, ring, iron core
Application: How does a speaker work?
Faraday’s Law and Electrical Power Generation
Faraday’s law is the key concept that is used to generate more than 95% of all electrical
power in the world. From the diagram below, only a small amount of the Other
Renewable energies (mainly solar power) does not use Faraday’s law.
Let’s go back to the magnetic ring launcher again to explain Faraday’s law in detail. As
we have already stated, a current produces a B-field:
produces
currents 
→ B-field
Faraday’s Law essentially tells us how one can reverse this process, that is, how one
can start with a magnetic field and produce a current.
Faraday Law
Any change in the magnetic environment of a coil of wire will cause a voltage to be
"induced" in the coil. No matter how the change is produced, the voltage will be
generated. The change could be produced by changing the magnetic field strength,
moving a magnet toward or away from the coil, moving the coil into or out of the
magnetic field, rotating the coil relative to the magnet, etc. To be more specific, a timevarying B-field will produce a time-varying voltage (or current):
Lecture 8-5
produces
time-varying B-field 
→ time-varying voltage



time-varying current
PhET Applet Faraday’s Law (choose pickup coil): http://phet.colorado.edu/en/simulation/faraday
DEMO Galvanometer, coil, magnet
Key points:
• If the magnet is moving towards (or away) from the coil, a voltage/current is induced
in the coil as read by the galvanometer’s needle moving.
• If the magnet is stationary, there is no voltage/current in the coil since the
galvanometer has no reading.
• Only changes time-varying changes in the B-field strength inside the bore of the coil
will produce a voltage/current in the coil.
Power Plants
Where do we get the electrical power for our homes and industries? These are
generated at power plants. There are many different kinds of power plants but the basic
structure is roughly the same. Consider the following diagram
This is a diagram of the workings of a boiling water reactor (BWR). In a BWR, the core (yellow
columns) is suspended in water (blue). The heat produced by the nuclear reactions boils the
water into steam (red). This turns a turbine (green), which drives an electricity generator (grey).
The steam then passes through a condenser, which uses water from a cooling pond (lower right),
before being passed back into the reactor chamber. The water acts as both the reactor coolant
and the moderator. Control rods (blue, between core columns) can be raised or lowered to control
the reaction. This is a cheap and simple reactor.
Electrical generator core (coil)
Only the outer part of the core is seen here, consisting of numerous
electrical wires arranged around the central space. When in use, a
turbine (not seen) will drive a shaft that runs through the central
space. Magnets on the shaft will rotate as the shaft rotates, inducing
electrical currents in the wires. This electrical current is then
distributed around a grid and used to power electrical devices. The
rotation of the turbine is usually by steam in a coal or nuclear power
station, but other types of power include wind, geothermal, and
hydroelectric power.
Lecture 8-6
PhET Applet Faraday’s Law (choose generator): http://phet.colorado.edu/en/simulation/faraday
Hydroelectric Power
Hoover Dam on the Colorado River and a Prototype wave-powered electricity generator in
Scotland (supply electricity to small, isolated communities).
Wind Power
Heat Power: Odeillo-Font-Romeau solar power station in the Eastern Pyrenees, France
Lecture 8-7
Positioned in front of the reflector is an array of 63 flat orientating mirrors that automatically track
the motion of the sun. The reflector is composed of 9,500 mirrors which concentrate the sun's
rays onto a dark-coated furnace at its focus (central tower). This system is capable of achieving a
o
temperature of 3,800 C.
Sewage Power (England)
Generators obtain electricity from burning methane gas produced from sewage (England).
Smog Power (Proposed tower design to remove smog from air whilst making power)
The tower is made from steel columns (red) with Teflon-coated fiberglass (white) stretched
between them. It is up to 60 stories high & 150 m wide. A mist of charged water droplets is
sprayed into the top of the tower. These droplets attract & absorb smoke particles via charge
attraction. Some of the droplets evaporate; cooling the air and making it sink into the tower. As
the air exits at the bottom of the tower it passes through wind turbines; these generate more
electricity than is needed to pump the water up the tower.
Magnetic Force on a moving charge particle
There are two types of forces in nature: (i) forces that change the speed of the particle
but not its direction, while the (ii) second kind of force changes the direction of the
particle but not its speed. Magnetic forces are the second kind of force: they change the
direction of a charge particle but not its speed. That is, magnetic forces cause
electrically charged particles to turn in circles.
Lecture 8-8
Magnetic forces are the second kind of force: they change the direction of a charge
particle but not its speed. That is, magnetic forces cause electrically charged
particles to turn in circles. The equation that for the magnetic force is complicated
enough for me not to go into details; however, there are two things to note for our
purposes. The magnetic force depends on the velocity of the charge particle as well as it
direction.
Velocity dependence
=
a stationary charge (v 0) feels no
=
B-force

→ FB 0
a moving charge (v ≠ 0) does feels a B-force 
→ FB ≠ 0
Direction dependence
If a charged particle is moving parallel or antiparallel to a magnetic field, there is no
B-force (no deflection force) acting on the charge particle.
If a charged particle is moving perpendicular to a magnetic field, there is a maximal
B-force (deflection force) acting on the charge particle. When this is the case, the Bforce will make this electric charge move in a circle such that it encircles the magnetic
field lines.
If a charge particle enters the magnetic field at an angle, then the charge particle will
have a helix trajectory.
DEMO TV with magnet
Why is the Earth’s B-field Important?
Lecture 8-9
The geomagnetic field and the van Allen Belts deflect the solar wind using a magnetic
force.
The white lines represent the solar wind; the purple line is the bow shock line; and the
blue lines surrounding the Earth represent its protective magnetosphere.
CME = Coronal Mass Ejection
The Sun’s magnetic field and releases of plasma directly affect Earth and the rest of the
solar system. Solar wind shapes the Earth’s magnetosphere and magnetic storms are
illustrated here as approaching Earth. These storms, which occur frequently, can disrupt
communications and navigational equipment, damage satellites, and even cause
blackouts. The magnetic cloud of plasma can extend to 30 million miles wide by the time
it reaches earth.
Lecture 8-10
Northern and Southern Lights
It is caused by interactions between charged particles from the Sun (the solar wind) and
gas atoms and molecules about 100 km above the Earth. On reaching Earth, the
charged particles are drawn by Earth's magnetic field to the poles, where they collide
with gas atoms and molecules, causing them to emit light.
Why aren't permanent magnets really permanent?
The north pole of a compass is attracted to the north pole of the Earth, yet like poles
repel. Can you resolve this apparent dilemma?
Lecture 8-11