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Electricity and Magnetism ... Chapter 12 – PowerPoint PPT presentation

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Title: Magnetism

  • Chapter 12

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.
  • 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.

  • 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.

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.

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
  • Where the lines are closer together, the field is

  • 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
  • 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.

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

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

  • 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.

  • 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

  • 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.

  • 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
  • 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

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
  • Can travel vibration-free at high speeds because
    they make no physical contact with the guideway
    they float above.

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.

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

  • 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)

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

How does an electric motor work?
  • In an electric motor the current is made to
    change direction each time the coil makes a half

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

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

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
  • The voltage is caused, or induced, by the
    relative motion between a wire and a magnetic
  • 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
  • The amount of voltage induced depends on how
    fast the magnetic field lines are entering or
    leaving the coil.

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
  • Three ways in which voltage can be induced in a
    loop of wire (changing magnetic field in the
  • 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)

(No Transcript)
LEFT HAND RULE (Flemings Rule)
  • The direction of Induced Current in a motor will
    always move in a direction according to the LEFT
  • If the thumb, forefinger, and middle finger of
    the left hand are held at right angles to each
  • Thumb is pointing in the direction the wire is
  • 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

Power Production
  • A generator is used to convert mechanical
    energy into electrical energy by electromagnetic
  • 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,
  • The rotating loops of wire in the armature cut
    through the magnetic field of the surrounding
    electromagnets, inducing alternating voltage and

Fossil Fuels
Hydro Dam
Hydroelectric power
Nuclear power
Wind Power
Tidal Power
  • Solar water heating
  • Photovoltaic cells
  • Solar furnace

  • 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
  • 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