Chapter 24 Magnetism

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Chapter 24Magnetism
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WARNING

• Strong magnets will be passed around the room.
• Keep these magnets away from iPods and laptops
  (especially my laptop)!

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Magnetic Forces

• General observations regarding magnets
• Iron (and a few other metals) are ferromagnetic,
  which means they can become magnetized.
• Magnets attract ferromagnetic metals.
• Two magnets can either attract or repel each
  other depending on poles.

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Magnetic Poles
Cow magnets

• Two types of magnetic poles
• North (N) and South (S)
• As with electric charges, like poles (NN, SS)
  repel and opposites (NS) attract.
• Unlike electric charges, cannot have just a North
  or just a South pole

N
S
Magnets N S on sides
Compass needle is magnet
Magnetic poles are not electric charges
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Demo Magnetic Fields

• Magnetic field points from South to North.
• Iron filing act as tiny compass needles,
  outlining magnetic field lines.

Magnet
Inside the magnet, field lines go North to South
Iron filings in clear oil
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Ferromagnetic Metals

• Ferromagnetic metals have similar atomic
  structure.

Spin of the electron in these metals produces a
net magnetic field
Iron, Cobalt, Nickel
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Demo Magnetism Money
Most US coins are not made of ferromagnetic
materials but many other countries use iron steel
in their currency.
Some pennies were made of steel during World War
II
Some Euro coins contain steel
Buffalo nickels are 25 nickel metal, which is
ferromagnetic
Iron is in the ink used in US paper currency to
avoid counterfeiting.
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Magnetic Domains

• The magnetic field of an single iron atom is so
  strong that interactions among adjacent atoms
  cause large clusters of atoms, called magnetic
  domains, to line up with one another.

A microscopic view of magnetic domains in a
crystal of iron. Each domain consists of billions
of aligned iron atoms. The blue arrows pointing
in different directions tell us that these
domains are not aligned.
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Magnetizing Iron

• Magnetic domains can be induced to align by an
  external magnetic field.

S N
Strong Magnet
N
S N
Strong Magnet
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Demo Magnetizing Iron

• Magnetic domains in iron nails are induced to
  align by proximity of the strong magnet
• Each nail becomes itself a magnet, which in turn
  magnetizes the nail below it, forming a chain.
• When the strong magnet is removed, most of the
  domains un-align and nail lose most of their
  magnetization.

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Demo Demagnetizing Iron

• Magnetic domains can be scrambled by heating the
  iron, striking it with great force, or other
  disruptions of alignment.

Magnetized
Demagnetized
Test tube of un-magnetized iron filling
S
SHAKE
N
S N
Magnetizer
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Demo Demagnetizing Iron
Iron nail is attracted to the large magnet due to
alignment of domains in the nail. Heat the nail
to a high temperature and the domains become
randomized so the nail is no longer attracted to
the magnet.
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Demo Electric Currents Magnetic Fields

• An electric current produces a magnetic field.

Electric Current
Magnetic Field
Field lines point in the northward direction
never reach a North Pole
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Electromagnets

• Electric current in a coil of wire creates a
  magnetic field similar to a bar magnet.

N
Current passing through loops of coiled wire
S
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Demo Electromagnets

• Electromagnet created by passing current through
  a coil of wire.
• Electromagnet is stronger when an iron bar is
  inserted within the coil.

N
Iron Bar
Note Do this in lab too.
Wire Coil
S
Used in lab
Connect to battery or power supply
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Check Yourself

• When an object is charged with static
  electricity, is that object also magnetized?
• But isnt iron is magnetized when the electrons
  are aligned. If iron has electrons, then why
  isnt it charged?

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Loudspeakers
Loudspeaker has a membrane but oscillations are
created by variations in electrical current,
which cause an electromagnet to be pulled towards
and away from a second, permanent magnet.
These oscillations cause the membrane of the
loudspeaker to vibrate with the same frequency as
the oscillations in the electrical current.
Headphones work essentially the same way, theyre
just smaller.
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Earths Magnetic Field

• The configuration of the Earth's magnetic field
  resembles a strong bar magnet located near the
  center of the Earth.
• The magnetic South pole is near the geographic
  North pole (so North pole of compass attracted
  towards Northern direction).
• There isnt an giant underground magnetized chunk
  of iron Earths interior is simply too hot.

N S
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Origin of Earths Magnetic Field

• Earth is an electromagnet with electric currents
  deep below the surface. Moving charges, looping
  around within the molten part of the Earth,
  create the magnetic field.

These currents are possibly the result of thermal
convection rising from the central core combined
with the rotation of the Earth about its axis.
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Demo Magnetic Force Current

• Moving charges in an electric current experience
  a force due to magnetic field.

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Electric Meters

• Since magnetic force on a wire depends on the
  current, can use this effect to design a meter to
  measure current (an ammeter).

Current
Needle moves
Current
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Magnetic Force on Charges

• Moving electric charges deflected by magnetic
  fields.

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Demo Crookes Tube
Electron beam in a Crookes tube is deflected
when a magnet is brought near the tube.
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Television Tube

• Electron beams, deflected by magnetic fields, are
  used to create TV images.

Electromagnets
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Demo Magnets TV sets
Picture on a TV set is distorted by presence of a
magnet since picture formed by an electron beam.
Warning Strong magnets can permanently damage a
television or a computer.
N S