chp. 21 magnetism
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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: - PowerPoint PPT PresentationTRANSCRIPT
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Chp. 21 Magnetism
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
B is a magnetic field vector The test object is taken to be a charged particle moving with a velocity v
**No Velocity = No Force **Units: B is measured in Tesla (T)
1T = Webers/m2 = 1Ns/Cm= 1x 104 Gauss (cgs unit)
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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 = BsinθIl
I is the current
l is the length of the wire
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Strength of the Magnetic Field
Plus Examples: 21A and 21 B pg. 774 & 778
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Force on a Current Carrying Wire
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Force on a Charged Particle
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• 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 N2. B = 7.4 x 10-5 T4. I = 1.34 A
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Right Hand Rules: First 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.
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Second 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.
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Third Right Hand Rule• 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.
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Magnetic Field Definitions
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Forces on Currents in Magnetic Fields• When you have a
current in a magnetic field it uses the third right-hand rule to show the direction of the force.
• The equation for this example is: F = BIL
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Example of Current Carrying Wire• A good example of how we
utilize a current carrying wire is in a loudspeaker. As the levels of electrical signal changes, it causes moderating amounts of magnetic force that moves the speaker cone. The speaker cone compresses air into sound waves.
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Torque on a Current Carrying Loop
• The torque due to a magnetic field B on a current carrying loop of area A is : = BIA sin
• This is the basis of volt and ammeters.
• If there are more than one loop of wire the equation becomes: = NIAB sin
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Induced EMF
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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.
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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).
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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.
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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
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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.
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Magnetic Fields in a Solenoid
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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).