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Chp. 21 Magnetism

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Title: Chp. 21 Magnetism


1
Chp. 21 Magnetism
2
MAGNETS
  • Magnets are pieces of metal (iron, nickel and
    steel) that work
  • according to rules similar to electric charges.
  • All magnets have 2 poles, north (north seeking),
  • and south poles.
  • Like electrostatics
  • similar poles repel and dissimilar poles
    attract.

3
History of Magnetism 1
  • Pierre de Maricourt mapped out and found "poles"
    on a spherical magnet in 1269. This was the first
    encounter with the well known electrostatic
    principals of like charges (poles) repel each
    other and opposite charges (poles) attract.

4
Magnetic History - 2
  • In 1600 William Gilbert extended these
    experiments
  • to a variety of materials. He even found that
    the
  • earth was a permanent magnet with a magnetic
  • force field. He concluded that poles always
    appear in
  • pairs and that magnet poles cannot be isolated.

5
Magnetic History -3
  • In 1819 Hans Oersted found that an electric
    current
  • in a wire deflected a nearby compass needle.
  • Andre Ampere deduced the quantitative laws of
  • magnetic force between current carrying
    conductors.

6
Magnetic History - 4
  • In the 1820's, Joseph Henry and Michael Faraday
    showed that an electric current could be produced
    in a circuit by either moving a magnet near the
    circuit or by changing the current in another
    nearby circuit. These observations demonstrated
    that a changing magnetic field produces an
    electric field.
  • However there was no
  • quantitative explanation
  • until Maxwells Equations.

7
Magnetic History 5
  • 1864- James Clerk Maxwell was able to show
  • that electricity and magnetism are two
  • perpendicular aspects of the same thing in his
  • unified theory of electromagnetism. He
    published
  • his 4 mathematical equations that related all of
  • electricity and magnetism through calculus.

8
Different Magnetic Materials
  • Materials that are not affected by magnetic
    forces
  • (non-magnetic) are called diamagnetic.
  • Materials that are affected by a magnetic field
  • (temporary magnets) are called paramagnetic.
  • Materials that produce or retain their magnetism
  • (permanent magnets) are called ferromagnetic.

9
Temporary Magnetic Materials- Paramagnetism
  • Paramagnetism occurs in substances in which the
  • atoms contain unpaired electrons.
  • This is common in most metals that are not
    permanent magnets. Example paper clips.

10
Permanent Magnetic Materials -
Ferromagnetism
  • Ferromagnetic materials contain clusters of atoms
    that
  • all have their unpaired electrons aligned
    (domains)
  • and produce a magnetic field.
  • These are permanent magnets.

11
Magnetic Fields
  • Magnetic Fields are like electrical and
    gravitational fields,
  • they produce forces on the surrounding area that
    drops
  • off as you move away from the magnet.
  • The vector arrows move out of the north end and
    curl around
  • to the south end. The biggest magnet in the
    world is the
  • Earth itself.

12
The Magnetic Force
  • The MAGNETIC Force acting on a charge q
  • moving with a velocity v in an external
  • magnetic field B is given by
  • Fmagnetic q v B
  • q v sin? B
  • No Velocity No Force
  • Units B is measured in Tesla (T)
  • 1T Webers/m2 1Ns/Cm 1x 104 Gauss (cgs unit)

13
Magnetic Force on a Current Carrying Conductor
  • For a current in a conductor,
  • we have charges in motion.
  • The force of a magnetic field on a wire is a
    summation of
  • the forces on the individual charges moving
    through the wire.
  • Fmagnetic BIl
  • B(sin?)Il
  • I is the current
  • l is the length of the wire

14
Strength of the Magnetic Field
  • Plus Examples 21A and 21 B pg. 774 778

15
Force on a Current Carrying Wire
16
Force on a Charged Particle
17
  • Hmwk. Chp. 21 BK and WKBK (11)
  • Book pg. 775 1,3,5
  • pg. 778 1,3
  • WKBK
  • 21A 1. F 5.4 x 10-11 N
  • 2. F 3.6 x 10-6 N
  • 4. B 2.6 T
  • 21B 1. F 0.23 N
  • 2. B 7.4 x 10-5 T
  • 4. I 1.34 A

18
Right Hand Rules 1st Right hand Rule
Current produced Magnetic Field
  • A series of right hand visualizations are
    possible to help you understand magnetism.
  • The first one is to describe the direction of
    magnetic field lines around a current carrying
    wire.

19
2nd Right Hand Rule- Electromagnet Polarity
  • The direction of the field produced by an
    electromagnet
  • can be found by using the Second Right-Hand
    Rule.
  • Curl your fingers around the loops in the
    direction of the
  • conventional (positive) current flow. Your
    thumb points
  • toward the North pole.

20
3rd Right Hand Rule Finding Magnetic Flux
  • The easy way to see this 3 way mutually
    perpendicular
  • component is the second right hand rule. The
    velocity of
  • charges, magnetic flux (B) and the force are
    each 90o from
  • the other.

21
Magnetic Field Definitions
22
Induced EMF
23
Motion of a Charged Particle in a Magnetic Field
  • The force of a charged particle is perpendicular
    to both the field and velocity and therefore a
    center seeking circle force (centripetal) equal
    to
  • F qvB mv2 /r
  • and thus r mv/qB
  • showing the radius is proportional to the
    momentum mv.

24
Magnetic Field of a Long Straight Wire
  • The direction of B around a wire is consistent
    with the first right-hand rule grasp the wire
    with the right hand and the thumb pointing in the
    direction of the current the fingers will point
    in the direction of the magnetic field lines.
  • The strength is found with B ?oI 2?r
    where r is perpendicular distance from
    the wire to the point and ?o is the
    permeability of free space (4? x10-7 (Tm/A).


25
Magnetic Force Between Two Parallel Conductors
  • The magnitude of the magnetic field around a long
    straight wire is determined to be B ?oI 2?d
    where d is distance.

26
Magnetic Field of a Current Loop
  • The magnetic field produced by a single, circular
    loop of wire looks similar to that produced by a
    short dipole magnet

27
Magnetic Field of a Solenoid
  • A solenoid is a long wire wound in the form of a
    helix.
  • Tightly wound solenoids produce a very strong
    magnetic field
  • inside of the loops. The strength depends on the
    number of
  • loops of wire. Solenoids are used widely in
    switches.

28
Magnetic Fields in a Solenoid
29
Induced Electrical Current
  • Just like moving charges produce a magnetic
    field. A moving magnetic field can produce an
    Induced Electrical Current.
  • Faradays Law of induction related magnetic flux
    change to the electromotive force (emf) or
    potential electrical change (voltage).
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