Magnetism Chap'14

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MagnetismChap.14

• A super-cooled superconductor is used to create a
  strong electromagnet, levitating an object into
  the air.

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14-1 Magnets - got their name from Magnesia (part
of Greece). They were called lodestones about
3000 years ago.

• Natural magnets are made of magnetite (a
  naturally-occurring mineral).
• Permanent magnets - magnetic all the time. Can be
  made by stroking iron with a strong magnet.
• Heating and hammering will destroy magnetism.
• Magnetically hardretains magnetism well. Cobalt
  and nickel (hard to magnetize, but hard to
  demagnetize as well.

Magnetically softeasily magnetized, but
easily demagnetized, too. Iron
Whats wrong with this picture?
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• Only certain metals can be magnetized. Fe, Co,
  Ni, Barium or Strontium ferrite, etc.
• A magnets field affects nearby metals like the
  above, temporarily magnetizing them and causing
  them to act as magnets themselves.
• Magnets exert magnetic forces of attraction or
  repulsion on each other.
• Magnetic poles - areas at either end of magnet
  where force appears to be the strongest.
• Each magnet has a pair of poles, a north pole and
  a south pole.

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• Like poles repel, unlike poles attract.
• A physical force of push or pull occurs as poles
  approach each other.
• Cant separate polesif magnet is cut, each piece
  will have a N and S pole.
• Magnetic poles have force because of how their
  magnetic fields interact (see right).
• Magnetic field - region where a magnetic force
  can be detected.

Magnetic field
N S
The great artwork of Mr. G!
N S
N S
attract
N S
S N
repel
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• Magnetic field lines circle around in an arc from
  one pole to another. A compass needle will trace
  out this field if moved near it.
• Magnetic field is strongest near poles and weaker
  further away.
• A compass is a magnet free to turn. It aligns
  itself with Earths magnetic poles (like iron
  filings near a bar magnet).
• Earth acts like a giant bar magnet (it has a mag.
  field).

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• Earths magnetic field has direction and
  strength. The north magnetic pole (in NE Canada
  near geographic north) is actually a magnetic S
  pole, and the opposite pole is in Antarctica.
  (Recall, N pole of compass is attracted, and
  opposites attract).

Source debated. Field has reversed over 20X
in last 5 million years of time (sea-floor rocks
record this).
Earths Magnetic field around earth makes ions
coming from the sun glow (Aurora borealis - the
northern lights).
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14-2 Electromagnetism-a strong magnet created
when an iron core is inserted in the center of a
current-carrying solenoid.

• 1820-Hans Oersted found that a current-carrying
  wire moves a compass.
• When current moves in a wire, a magnetic field
  wraps around it.
• Right hand rule grasping wire with thumb
  pointing to , fingers show how mag. field wraps
  around wire.
• If wire is wrapped in a coil, the field is
  strengthened around that point.

Current direction
Magnetic field wraps around wire like
hands fingers...
Solenoid- a long, wound coil of insulated wire.
Current through wire will produce a magnet
effect as long as the current is on.
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• Solenoid power increased by iron core that makes
  magnetic field stronger (electromagnet).
• Electrons spin. This makes a tiny magnetic
  field. Most materials cancel out w/e- of
  opposite spins. Some have some e- that dont
  cancel (Co, Ni, Fe).
• Electrons tend to spin in small groups called
  domains (a microscopic magnetic region of a
  grp.of atoms w/magnetic field aligned in the same
  direction. In a piece of iron, for example,
  these domains are usually random in direction and
  still cancel each other out. If a strong
  magnetic field occurs near them, they tend to
  shift and line up with it, producing a magnetic
  field b/c more of them are spinning in a
  particular direction.
• See fig.14-9, p.471. For examples of this...

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Intermission

• A little light humornote differences on next
  slide.

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just a little fun with Photoshop, and...
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Electromagnetic devices--lots of things use
electromagnets to perform tasks. The ability to
use electricity to create a force is very useful.
Examples include

• Galvanometer- device that measures amount of
  current in a device.
• A coil of insulated wire is suspended between 2
  magnet poles. When current runs through the
  wire, the magnetic fields interact and twist the
  coil, moving a needle to show the current flow
  occurring.
• See p.471 and lab activity

N
S
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• Electric Motor - a device that converts electric
  energy to mechanical energy.
• A coil of wire between two magnet poles with
  electricity flowing through it is attracted more
  to one pole, so it turns. A device called a
  commutator reverses the current. Thus the coil
  is now attracted to the other pole, and turns
  around again. This process repeats over and over
  like a dog chasing its tail. (see p.472)

S
When the spindle turns, the current goes to the
other copper plate and reverses direction of
current.
N
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• Stereo speakers use electric/magnetic force.
• A magnet is placed around a coil of wire at the
  base of a paper cone (see left).
• When electric signals go through the coil, The
  force generated by the interaction with the
  nearby magnetic field sets up a vibration.
• This vibratingsound from the paper cone-- means
  electricity has been turned into sound by mag.
  force.

S
S
N
Different kinds of speakers have different
magnets, currents, resistances, and paper cones
to make different levels and pitches of sound.
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14-3 Electromagnetic Induction-making electric
current by moving wire in a magnetic field.

• 1831 M.Farraday discovered current can be
  produced by pushing a magnet through a coil of
  wire.
• Farradays Law An electric current can be
  produced in a circuit by a changing magnetic
  field.
• We say a current is induced. Not conducted from
  somewhereits madefrom the force of the motion.
  (see diagrams on p.474 and 476).
• Generator- device that uses electromag.induction
  to convert mechanical en. into electric energy.

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• Generators are made like motors, but they are
  designed to turn mechanical energy into
  electrical energy instead of the other way
  around.
• The loop of wire turns (with engine power, water
  power, etc. and produces current.
• B/c the direction determines current flow, each
  1/2 turn

of the wire the current reverses
directions. This is called alternating current
(AC). House current is alternating current.
(see Table 14-1, p.477 for details).
2nd 1/2 circle, current flows other way
thru wire...
1st 1/2 circle, makes current flow through the
wire one way
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• Generators are very important! They produce the
  power you use in your home from mechanical
  energy. Sources hydroelectric, coal-fired,
  wind, solar, hot geothermal water, and nuclear
  fission.
• Where electricity is, magnetism is, and vice
  versa. They are actually two parts of the same
  thing.
• Electromagnetic waves (such as light, radar,
  radio, microwaves, and X-rays) are electric and
  magnetic waves moving through each other at right
  angles. El. is part of it, and mag. is part of
  it. (see fig.14-16, p.478).

Magnetic component
900
Electromagnetic wave
Electric component
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Transformers-device that changes one AC voltage
to a different AC voltage.

• Often seen as large cylinders on utility
  polesthey are lowering the voltage into your
  house to the proper 110V AC.
• The wire on Primary produces a changing mag.
  field that induces current on sec. wire w/o
  direct conduction.
• Amount of voltage in proportion to the number of
  turns on either side.

Prim. 4 turns
Secondary 2 turns
1/2 volts
(p.479 has more examples)
Transformer on a downed utility pole
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• Tranformers with more secondary coils than pr.
  will produce higher voltage. These are called
  step-up transformers.
• Transformers with fewer sec. loops than pr. loops
  will lower voltage and are called step-down
  transformers.

Which type is on a pole near your home?
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The End

• of some leftover metal stuff anyway. What is
  this thing one heck of a huge example of?