Magnetism

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Magnetism

• Chapter 12

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What is Magnetism?
Magnetism is the force of attraction or repulsion
of a magnetic material due to the arrangement of
its atoms, particularly its electrons.
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• Magnets?
• The earliest magnets were found naturally in the
  mineral magnetite which is abundant the rock-type
  lodestone. These magnets were used by the ancient
  peoples as compasses to guide sailing vessels.
• A magnetic field is produced by the motion of
  electric charge. Two kinds of electron motion
  make magnetism
• Electron spin electrons spin about their own
  axes like tops
• Electron revolution electrons revolve about
  the atomic nucleus like planets revolving around
  the sun
• In most common magnets, electron spin is the main
  contributor to magnetism.
• Every spinning electron is a tiny magnet.
• A pair of electrons spinning in the same
  direction makes a stronger magnet.
• A pair of electrons spinning in opposite
  directions, however work against each other. The
  magnetic fields cancel. This is why most
  substances are not magnets.

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• Magnets? (cont)
• Iron, Nickel, and Cobalt are materials that are
  easily magnetized because their fields do not
  cancel each other entirely.
• Most common magnets are therefore made from
  alloys containing iron, nickel, and cobalt in
  various proportions.
• Some materials are easy to magnetize, but they
  lose their magnetism quickly these are called
  temporary magnets.
• Materials that are more difficult to magnetize,
  but they tend to stay magnetized, are called
  permanent magnets.

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Magnets have two ends or poles, called north and
south poles.

• Unlike poles of magnets attract each other and
  like poles of magnets repel.
• Electric Charges can be isolated, but Magnetic
  Poles cannot.
• If you cut a magnet in half you get 2 Magnets
  both with a North and South Pole
• At the poles of a magnet, the magnetic field
  lines are closer together.

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Magnetic Fields

• A magnetic field is produced by the motion of
  electric charge.
• The region where the magnetic forces act is
  called the magnetic field
• The filings line up with the magnetic field lines
  that spread out from one pole and return to the
  other.
• Where the lines are closer together, the field is
  stronger.

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• The earth is like a giant magnet!
• The nickel iron core of the earth gives the earth
  a magnetic field much like a bar magnet.

Magnetic South Pole

• The needle of a compass always points toward
  the magnetic south pole.

• We call this direction North (remember,
  opposites attract)

Magnetic North Pole
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• What are magnetic domains?
• Magnetic substances like iron, cobalt, and nickel
  are composed of small areas where the groups of
  atoms are aligned like the poles of a magnet.
• These regions are called domains.
• All of the domains of a magnetic substance tend
  to align themselves in the same direction when
  placed in a magnetic field.

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How to weaken a magnet

• Drop it
• Heat it
• Hit it with a hammer
• Some of the domains are jostled out of alignment
  so they become random again

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• Electricity and Magnetism how are they related?
• A single moving charge produces a magnetic
  field.
• When an electric current passes through a wire
  a magnetic field is formed.

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LEFT HAND RULE

• We can use the fingers and thumb of the left
  hand to determine either the direction of Current
  Flow or the direction of the Magnetic Field or
  direction of the Magnetic North Pole
• Conductors
• Thumb pointing in Direction of Current Flow
• Fingers are pointing in the direction of the
  magnetic field.
• Electromagnets
• Fingers pointing in Direction Current Flow
• Thumb points in Direction of North Pole.

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• What is a Solenoid?
• A long coil of wire with many loops in it is
  called a solenoid.
• Magnetic field lines about a current-carrying
  wire crowd up when the wire is bent into a loop.
• More loops mean more magnetic field intensity.
• In summary then a Solenoid acts as a Magnet
  when a Current passes through it
• Three things that can strengthen the magnetic
  field of a solenoid
• Increasing the of Coils
• Increasing the Current (Amps) flowing through it
• Placing a piece of Iron in the Center of it
  (Electromagnet)

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• What is an electromagnet?
• When an electric current is passed through a
  coil of wire (solenoid) wrapped around a iron
  core, the alignment of magnetic domains in the
  iron produces a very strong magnetic field. This
  is called an electromagnet.

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• What are the differences between an
  electromagnet and a permanent magnet?
• You can make an electromagnet stronger
• Increase the current (amps)
• You can change the direction of the poles in an
  electromagnet
• Change the direction of current
• You can turn an electromagnet on and off
• Close or open the circuit (flip a switch)
• An electromagnet would then be classified as a
  temporary magnet

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Superconducting Electromagnets

• Ceramic superconductors have the interesting
  property of expelling magnetic fields. Because
  magnetic fields cannot penetrate the surface of a
  superconductor, magnets levitate above them.
• Used for the levitation of high-speed trains for
  transportation.
• Can travel vibration-free at high speeds because
  they make no physical contact with the guideway
  they float above.

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Magnetic Forces are Exerted on Moving Charges

• A charged particle has to be moving to interact
  with a magnetic field (charges at rest dont
  respond to magnets)
• The force is greatest when the particles move at
  right angles to the magnetic field lines (at
  other angles, the force is less and becomes zero
  when they move parallel to the field lines)
• The force is always perpendicular to the magnetic
  field lines and perpendicular to the velocity of
  the charged particle.
• So a moving charge is deflected when it crosses
  through a magnetic field, but when it travels
  parallel to the field no deflection occurs.

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Forces Gravitational, Electrical, Magnetic

• Gravitation force act in a direction parallel to
  the line between masses
• Electrical force acts in a parallel direction
  between charges
• Magnetic force acts at right angles to the
  magnetic field and the velocity of the charged
  particle

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• What is a galvanometer?
• A galvanometer is an electromagnet that interacts
  with a permanent magnet.
• The stronger the electric current passing through
  the electromagnet, the more it interacts with the
  permanent magnet.

• Galvanometers are used as gauges in cars and many
  other applications such as electric meters
  (ammeter, voltmeter)

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• What are electric motors?
• An electric motor is a device which changes
  electrical energy into mechanical energy.

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How does an electric motor work?

• In an electric motor the current is made to
  change direction each time the coil makes a half
  rotation.

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• Current in a motor is reversed during each half
  revolution by stationary contacts on the shaft.
  These contacts are called Brushes.

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Larger Motors

• Larger motors, DC or AC, are usually made by
  replacing the permanent magnet by an
  electromagnet that is energized by some power
  source.
• Many loops of wire are wound about an iron
  cylinder, called an armature, which then rotates
  when the wire loops carry current.

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What is Electromagnetic Induction?

• We have seen how electricity can produce a
  magnetic field, but a magnetic field can also
  produce electricity! How?
• In 1831 two physicists, Michael Faraday and
  Joseph Henry both discovered that electric
  current can be produced in a wire simply by
  moving a magnet in or out of a coiled part of the
  wire.
• The voltage is caused, or induced, by the
  relative motion between a wire and a magnetic
  field.
• Magnetic Field moves near a stationary conductor
• Conductor moves in a stationary magnetic field
• The greater the number of loops of wire moving
  in a magnetic field, the greater the induced
  voltage (10x as many loops induces 10x as much
  voltage)
• The amount of voltage induced depends on how
  fast the magnetic field lines are entering or
  leaving the coil.

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Faradays Law

• The induced voltage in a coil is proportional to
  the number of loops multiplied by the rate at
  which the magnetic field changes within those
  loops.
• Three ways in which voltage can be induced in a
  loop of wire (changing magnetic field in the
  loop)
• Moving the loop near a magnet
• Moving the magnet near the loop
• Changing a current in a nearby loop
• Examples of Electromagnetic induction
• Activation of traffic lights (car drives over
  buried coils of wire)
• Airport Security System (metal you carry alters
  the magnetic field in the coils)
• Magnetic strip on the back of a credit card is
  scanned (induced voltage pulses identify the card)

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LEFT HAND RULE (Flemings Rule)

• The direction of Induced Current in a motor will
  always move in a direction according to the LEFT
  HAND RULE.
• If the thumb, forefinger, and middle finger of
  the left hand are held at right angles to each
  other
• Thumb is pointing in the direction the wire is
  moving
• Forefinger is pointing in the direction of the
  magnetic field (north to south)
• Middle finger will point in the direction of the
  induced electron flow

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Power Production

• A generator is used to convert mechanical
  energy into electrical energy by electromagnetic
  induction.
• The generator has armatures made up of bundles of
  copper wires.
• The armatures are forced to spin within strong
  magnetic fields by a turbine, which is spun by a
  form of energy (falling water, steam, wind, heat,
  etc.)
• The rotating loops of wire in the armature cut
  through the magnetic field of the surrounding
  electromagnets, inducing alternating voltage and
  current.

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Fossil Fuels
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Hydro Dam
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Hydroelectric power
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Nuclear power
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Wind Power
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Tidal Power
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Geothermal
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Solar

• Solar water heating
• Photovoltaic cells
• Solar furnace

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Transformers

• Transformer device in which alternating
  current in one coil of wire induces a current in
  a second coil
• a transformer consists of two coils of wire
  around a magnet
• the first coil, called the primary coil, is
  connected to the power source
• the second coil, called the secondary coil, is
  connected to the load (something that uses
  electricity)
• a step-up transformer increases voltage
• it consists of more coils of wire in the
  secondary coil than the primary coil
• a step-down transformer decreases voltage
• it consists of more coils of wire in the primary
  coil than the secondary coil