Download Magnetism

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Worksheet by the door
History of Magnets
• More than 2000 years ago, rocks called
lodestones were found in the region of
Magnesia in Greece.
• In the 12th century, the Chinese used them for
navigating ships.
What are magnets?
• Most materials
– Have paired up electrons moving in opposite
directions.
– The field created by one moving charge is
canceled by the other.
– No magnetic field is created.
What are magnets?
• Any charges in motion
produce a magnetic field.
• Some materials like Iron, Nickel, or Cobalt
– Have a single electron or paired electrons spinning
in the same directions.
– The magnetic field created by one electron is not
canceled by the other.
– An atomic sized magnet is created.
As in electromagnetic waves, when an a charge moves it creates a magnetic
and electric field at right angles.
Why is Fe magnetic and Al not?
• What makes a good magnet?
– Every spinning electron is a tiny magnet.
– A pair of electrons spinning in the same direction is a stronger
magnet.
– A pair of electrons spinning in opposite directions work against
one another; the magnetic fields cancel.
• Fe has two unpaired electrons spinning in the
same direction.
– Cobalt has 3.
– Nickel has 4.
– Aluminum has one unpaired electron.
Magnetic Poles
• All magnets have two regions “poles” that produce
magnetic forces.
• They are named like this because if you take a
magnet and suspend it from the middle (so that it
can swing freely), it will rotate until the north pole
of the magnet points north and the south pole
points south.
• Like poles repel.
• Opposite poles attract.
No less than two.
• North and South cannot be separated.
• If a magnet is broken, poles aren’t separated;
two smaller magnets are formed.
Magnetic Domains
• In magnetic materials, neighboring atoms pair
up to form large groups of atoms whose net
spins are aligned.
• These groups are called domains.
• When a piece of iron is not a magnet, the
domains point in random directions.
Magnetic Domains
• If the non-magnetized iron is placed in a strong
magnetic field, the domains will line up in the
direction of the field.
• In temporary magnets, the domains will return to
their random orientation after the field is removed.
• In permanent magnets, the domains will remain
aligned.
• 1 domain = 1 quadrillion (1015) atoms
Magnetic Fields
• You have probably noticed that forces
between the magnets (both attraction and
repulsion), are felt not only when the magnets
are touching each other, but also when they
are held apart.
• In the same way that gravity can be described
by a gravitational field, magnetic forces can be
described by the magnetic fields around
magnets.
Magnetic Field Demo
• What kinds of magnetic fields are produced by
pairs of bar magnets?
Magnetic
Field Lines
The shape of the
magnetic field. If
you place a compass
in the field its arrow
will point parallel to
the field lines.
Magnetic Field Lines
• Magnetic field lines are the same as
electric field lines in that both are stronger
when lines are drawn closer together.
– So the magnetic field is stronger at the poles
• The magnetic field lines have arrows going
from north to south.
• Magnetic field lines do not cross because the
magnetic field cannot go in two directions at
once.
Temporary Magnets
• What happens when you place a magnet next
to a nail?
• This is because the magnet causes the nail to
become polarized; the nail becomes a magnet.
• Aluminum and lead are not a magnetic.
• This is temporary; if you pull the
magnet away, the nail loses its
magnetism.
Permanent Magnets
• Permanent magnets are produced in the same
manner as the nail; however, due to the
microscopic structure of the material, the
magnetism becomes more permanent.
• Most permanent magnets are made of ALNICO,
an iron alloy containing 8% Aluminum, 14%
Nickel, and 3% Cobalt. altho. Al is not a magnet
• Some rare earth elements, such as neodymium
and gadolinium, produce strong permanent
magnets.
Common Uses of Magnets
• Magnetic recording media: VHS tapes, audio cassettes, floppy disks, hard
disks.
• Credit, debit, and ATM cards
• Common television and computer monitors
• Speakers and Microphones
• Electric motors and generators
• Compasses
• Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips)
that are either too small, too hard to reach, or too thin for fingers to hold.
Some screwdrivers are magnetized for this purpose.
• Magnets can be used in scrap and salvage operations to separate
magnetic metals (iron, steel, and nickel) from non-magnetic metals
(aluminum, non-ferrous alloys, etc.).
• Magnetic levitation transport, or maglev, is a form of transportation that
suspends, guides and propels vehicles (especially trains). The maximum
recorded speed of a maglev train is 361 mph.
How to demagnetize a magnet
• Heating a magnet past its Curie temperature the molecular motion destroys the alignment
of the magnetic domains.
– 768°C for Iron
• Hammering or jarring –
the mechanical disturbance tends to
randomize the magnetic domains.
• Placing the magnet in an alternating magnetic
field.
Ferrofluid
• a mixture of tiny iron particles covered with a
liquid coating that are then added to water or oil.
• Used in car suspensions, cancer detection,
loud speakers
• video
Ferrofluid in Acura
Earth’s Magnetic Field
• Earth is a huge
magnet.
• This is possibly
due to the
molten Iron core.
• The magnetic
field around Earth
is called the
Magnetosphere
Earth’s Magnetic Field
• Magnetic north pole is different than geographical north pole.
• There is about a 25 ̊difference from geographic north pole to
magnetic north pole, this is called magnetic declination
• In addition, the north pole
of a magnet is attracted to
earth’s north pole because that
is the magnetic south pole.
• The south pole of a magnet
is attracted to the earth’s
south pole because that
is the magnetic north pole.
Magnetosphere
• Extends several tens of thousands of km into space.
• Protects Earth from solar winds.
Auroras
• Charged particles from the sun become
trapped in Earth’s magnetic field.
• This occurs near the magnetic poles.
• These charged particles collide with electrons of
the atoms in our atmosphere and transfer their
energy.
• The colors of the lights are determined by the type
of gases in the atmosphere.
– O2 releases green light; N2 releases red light
• aurora borealis (northern lights); aurora australis
(southern lights)
Dynamo Theory
• The dynamo theory proposes a mechanism by
which a celestial body such as the Earth
generates a magnetic field.
• In the case of the Earth, the
magnetic field is induced and
constantly maintained by the
convection of liquid iron in the
outer core.
Magnetic field of Earth is not stable
• The magnetic poles of Earth wander up to 15 km
every year.
• Based upon the study of lava flows throughout
the world, Earth's magnetic field reverses at
intervals, ranging from tens of thousands to
many millions of years, with an average interval
of approximately 250,000 years.
• The last reversal is theorized to
have occurred 780,000 years ago.
Other Planets’ Magnetic Fields
• The sun also has a strong magnetic field.
Evidence seen in sun spots.
– They occur in pairs.
– They also peak at an 11-year cycle, which
coincides with the flipping of the sun’s magnetic
field.
• Jupiter’s magnetic field is 10 times the
strength of Earth’s.
• The moon has no magnetic core, and hence,
no magnetic field.
Neutron Stars
• The most intense magnetic field ever found in
the universe has been observed around a
neutron star 40,000 light years from Earth.
• Neutron stars are compact objects that are
created during supernova explosions.
• The magnetic field of a Neutron Star is
estimated to be one thousand trillion times
the strength of Earth's magnetic field.
Animal Migration
• Some animal species do have the
ability to detect the magnetic field,
& they use it to make their migrations.
• Bats and sea turtles use magnetic
information to find their way.
• We're not 100 percent sure how animals detect the
magnetic field, but small particles of magnetite have
been found in the brains of some species. Those
particles may be reacting to the magnetic field and
activating nerves in such a way as to send orientation
information to the animal's brain.
Bacteria & Magnets
• Some bacteria have a chain of magnetite as part of
their internal structure
• They use this magnetite to find their way in swamps
• Bacteria in the northern hemisphere have magnetite
that are opposite in polarity than the bacteria with
magnetite in the southern hemisphere.
• Animal Migration Video
Moving Charges
• A moving charge produces a magnetic
field.
• Many charges in motion – an electric
current – also produce a magnetic field.
• This was discovered in 1820 by Hans
Oersted. He ran an electric current
through a wire and positioned compasses
around the wire.
As in electromagnetic waves, when an a charge moves it creates a magnetic
and electric field at right angles.
Electromagnetism
• Oersted discovered that a magnetic field can
be formed from a current, now we will learn
how a current can be formed from a magnetic
field
• When Michael Faraday made his discovery of
electromagnetic induction in 1831, he
discovered that a changing magnetic field is
necessary to induce a current in a nearby
circuit.
Creating Current
• Faraday discovered that if a wire is moved in a
magnetic field, a voltage is produced, and if
there is a complete loop, a current will flow.
This is how electricity is generated.
• This is electromagnetic
induction
Generating electricity
In industry, electricity is generated by spinning a
coil of wire in a magnetic field.
To increase the voltage or current generated:
• Spin the coil faster.
• Put more loops on the coil.
• Use a stronger magnetic field.
• Use a coil with a larger area.
It’s all related.
• So Oersted discovered that currents produce
magnetic fields, and 11 years later, Faraday
discovered induction.
• In the 1860s, Maxwell predicted that even
without wires, changing electric fields produce
magnetic fields, and changing magnetic fields
produce electric fields.
• The result of this was the discovery that
energy transmitted across empty space is in
the form of electromagnetic waves.
Why the loops?
• By bending the wire into a loop, the magnetic
field lines are bunched up inside the loop.
• If you add more loops, the magnetic field will
become stronger.
– Two loops = magnetic field is doubled.
– Three loops = magnetic field is tripled.
• Electromagnet – a current carrying wire with
many loops.
Add a core to the electromagnet
• If a piece of iron is placed inside the coil of the
electromagnet, the domains of the piece of
iron are forced into alignment, increasing the
intensity of the magnetic field.
• Electromagnets are strong
temporary magnets – they
can be turned on and off
easily.
Making an electro magnet in class. A magnet you
can turn on and off when you regulate the power
in the wire.
Applications of Induction
• Generators
– A generator is nothing more than the reverse
of an electric motor
– A motor, you put current (battery) into it ,
electrical energy, and you get mechanical
work out of it (spinning our wire)
– A generator, you put mechanical work into it
(water running, or falling, steam, etc.) and you
get current out
• Microphone
– A simple microphone is the reverse
(symmetry) of a speaker.
– A speaker, you put alternating current into it
and you get sound out.
– A microphone, you put sound into and you get
alternating current out.
Speakers
• Alexander Graham Bell patented the
first electrical loudspeaker as part of
his telephone in 1876.
• The loudspeakers in most sound
systems use magnets to produce
sound waves.
• One design is a flexible paper cone
attached to a coil of wire and a
permanent magnet
• The current through the wire causes a
magnetic force on the coil.
Transformers!
• Transformers can either step voltage up OR
down!
∙ This is the main reason why we use AC circuits!
• Notice that the transformer is made with two
different wire coils.
• The magnetic field of one coil is transferred to
the other coil via an iron core.
• The difference in coil configuration creates a
difference in voltage!
• The principal reason voltage is induced in the
loops of a generator coil is that loops are
rotating, and changing the amount of
magnetic field within the loops. V/n = V/n
Transformers
• You see transformers everywhere!
– They are those big cylindrical cans you see on
power poles
– They are those big green metal boxes you see on
the side of some buildings that say, “Danger! High
Voltage!”
– They are in many appliances that require to step
up the voltage supplied in our walls (120V)
– Because of transformers, AC voltage became the
voltage of choice