AP Physics- Magnetism
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Transcript AP Physics- Magnetism
Chp. 21 Magnetism
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.
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.
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.
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.
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 Maxwell’s Equations.
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.
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.
Temporary Magnetic MaterialsParamagnetism
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.
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.
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.
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 sinθ B
**No Velocity = No Force **
Units: B is measured in Tesla (T)
1T = Webers/m2 = 1Ns/Cm= 1x 104 Gauss (cgs unit)
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.
F
magnetic
l
= BIl
= B(sinθ)Il
I is the current
is the length of the wire
Strength of the Magnetic Field
Plus Examples: 21A and 21 B pg. 774 & 778
Force on a Current Carrying Wire
Force on a Charged Particle
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
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.
2nd Right Hand RuleElectromagnet 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.
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.
Magnetic Field Definitions
Induced EMF
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.
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 2r
where r is perpendicular distance from the
wire to the point and o is the permeability of
free space (4 x10-7 (Tm/A).
Magnetic Force Between
Two Parallel Conductors
The magnitude of the magnetic field
around a long straight wire is determined
to be B = oI 2d where d is distance.
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
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.
Magnetic Fields in a Solenoid
Induced Electrical Current
Just like moving charges produce a magnetic
field…. A moving magnetic field can produce an
Induced Electrical Current.
Faraday’s Law of induction related magnetic flux
change to the electromotive force (emf) or
potential electrical change (voltage).