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Magnetism

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Title: Magnetism


1
Magnetism
2
Magnets and Magnetic Fields
  • Naturally occurring
  • Attract or repel each other
  • Attract certain metals (iron, cobalt, nickel)
  • Point in a given direction relative to the Earth
    when allowed to hang freely
  • North and South poles- there are NO monopole
    magnets!
  • Opposite poles attract

3
  • Just as an electric charge is surrounded by an
    electric field, a moving electric charge is also
    surrounded by a
  • MAGNETIC FIELD.
  • Similar to how a planet orbits the sun and
    rotates on its own axis, an electron orbits the
    nucleus and spins on its own axis.
  • The MAGNETIC FIELD is due to both the orbital
    motion as the electron orbits the nucleus and the
    spin of each electron around its own axis

4
  • For elements like iron, cobalt, and nickel, all
    the electrons in those atoms spin in the same
    direction, instead of opposite directions, which
    would cancel the effect. Therefore, these atoms
    are naturally magnetic.
  • If the magnetic fields of large clusters of those
    atoms, called domains are aligned, the whole
    material is magnetized.
  • Not every piece of iron is a magnet. Only those
    pieces whose domains are all aligned!

5
  • Magnetic fields are created by
  • Magnetic materials ( those spinning electrons!)
  • But ALSO by
  • Current-carrying wires (a lot of moving charges!)
  • A magnetic field is represented by magnetic field
    lines- OUT of the north pole- INTO the south pole

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  • The Earth behaves like a giant bar magnet. This
    is due to the movement of Earths molten interior
    (lots of moving charges).
  • The magnetic poles do NOT line up with the
    geographic poles.
  • Under the North geographic pole lies a South
    magnetic pole!
  • The direction of Earths magnetic field is not
    rigidly fixed and moves around from year to year.
  • The poles have completely reversed numerous times
    (about every 100,000 years) according to
    geological evidence.

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9
Magnets and Current-Carrying Wire
  • A wire with current running through it produces a
    magnetic field.
  • Electromagnets (or solenoids) are magnetic only
    when current flows.

10
  • A MOTOR is a machine that converts electricity
    into mechanical motion. Most motors use
    electromagnets to produce rotation. A motor has
    a rotor spinning with coils of current carrying
    wires passing magnets.
  • Michael Faraday is credited with building the
    first electric motor

11
  • A generator does the reverse of a motor it
    converts mechanical motion into electricity!
  • Examples a wind or water turbine, steam engine
  • Who is credited with making the first generator?
  • Yes, Michael Faraday!

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13
Magnetic Forces
14
The Force on a Current-Carrying Wire in a
Magnetic Field
  • Current-carrying wires act like magnets.
  • Magnets exert forces on other magnets.
  • Therefore
  • Magnets exert forces on current-carrying wires.
  • F ILB
  • Where B is the magnetic field, measured in Tesla,
    T
  • I is the current, measured in amps
  • L is the length of the wire, measured in meters.
  • The current, I, and the magnetic field, B, must
    be perpendicular to each other.

15
The Direction of the Magnetic Force, F LIB
  • Force is a vector and always has a direction
    associated with it. The direction of the
    magnetic force is determined by a right-hand
    rule
  • Point your thumb in the direction the current is
    flowing.
  • Point your fingers in the direction of the
    magnetic field.
  • Your palm will face the direction of the magnetic
    force on the wire.

16
Speaker and Magnets
  • 1. There is a magnet in the back of most
    speakers.
  • 2. The wires going into the back of a speaker
    carry current from the amplifier.
  • 3. The coil of wire experiences a magnetic force
    which pushes it back a forth against the paper
    cone.
  • 4. The cone vibrates producing sound.

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18
The Flip-Side
  • Currents in wires
  • produce
  • magnetic fields.
  • AND
  • Changing magnetic fields
  • produce
  • currents in wires.

19
Michael Faraday
  • Michael Faraday studied both electricity and its
    relationship to magnetism.
  • We remember the Faraday cage that protects its
    contents from static charges. The unit for
    capacitance is the Farad.
  • He also made the first electric motor and
    generator and transformer!
  • Faradays Law states that a changing magnetic
    field will induce a current in a wire.

20
The metal detector
  • One application of Faradays law is the security
    metal detector at the airport. As a person
    passes through the detector, which is a large
    electromagnet, any metal on their person will
    change the magnetic field in the detector,
    inducing a current that signals the presence of
    metal!

21
Another application of Faradays Law The
Electric Guitar
  • The pickup in an electric guitar contains a
    coil of wire wrapped around a magnet.
  • The vibration of the guitar string causes the
    magnet to vibrate.
  • The changing magnetic field induces a current in
    the coil of wire.
  • The current is sent to the amplifier to produce
    the sound.

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23
The Force on a Moving Charge in a Magnetic Field
  • Just as current-carrying wires (lots of moving
    charges) experience a force in a magnetic field,
  • a single electrical charge also experiences a
    force if it is moving in a magnetic field.

e-
24
The Force on a Moving Charge in a Magnetic Field
  • The force is given by
  • F qvB
  • Where B is the magnetic field in Tesla,
  • q is the electric charge in Coulombs
  • v is the velocity in meters per second
  • Only perpendicular components of B and v
    contribute to the magnetic force.

25
CRTs and Magnets (TV and computer monitors- NOT
flat screen)
  • 1. Electrons are produced at the cathode.
  • 2. They move through an electromagnet.
  • 3. The electromagnet pushes them in different
    direction.
  • 4. The hit the phosphorescent screen, making it
    glow.

26
James Clerk Maxwell
  • Maxwell showed that electricity and magnetism
    were all part of the same phenomenon- the
    electromagnetic force! He wrote 4 very famous
    equations showing that relationship and
    describing electromagnetic waves

27
Particle Accelerators
  • This magnetic force is used to accelerate
    particles in laboratories like CERN, in
    Switzerland, to study fundamental particles and
    their interactions in its Large Hadron Collider.

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  • The magnetic force on a moving charge will be a
    centripetal force, causing the charge to move in
    a circular pathway.
  • Magnetic Force centripetal Force
  • qvB mv2 / r
  • The direction of the Force is given by the
    right-hand rule
  • Thumb v (velocity)
  • Fingers B (magnetic field)
  • Palm Force on a POSITIVE charge

30
Mass Spectrometer
  • A mass spectrometer is a machine that can
    determine the mass of a subatomic charged
    particle by measuring the radius of the circle it
    moves in while under the influence of a magnetic
    field.

31
qvB mv2 /r
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33
  • There are storms on the surface of the sun
    that are known as solar flares or sun spots.
  • These storms send massive amounts of cosmic
    radiation out toward space.

34
Earths Magnetic Field
  • The Magnetosphere acts like a shield around
    the Earth to protect us from cosmic radiation
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35
  • When the cosmic radiation is particularly
    intense, some of the rays penetrate the Earths
    Magnetosphere.

36
  • This radiation gets trapped in Earths
    magnetic field lines and can cause disruption of
    signal from satellites and can even cause the
    destruction of sensitive equipment inside
    satellites.

37
  • But it also can cause the atmospheric molecules
    near the field lines to glow.
  • This causes the auroras
  • Aurora Borealis, the Northern lights
  • Aurora Australis, the Southern lights.

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