MagnetismForce of Mystery

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Magnetism Standards

Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources. (Ch 36)

Students know how to determine the direction of a magnetic field produced by a current flowing in a straight wire or in a coil. (Ch 36)

Students know changing magnetic fields produce electric fields, thereby inducing currents in nearby conductors. (Ch 37)

Famous 19th Century Quote

“The nation that controls magnetism controls the Earth”

Magnetic Poles

North and SouthLike poles repel N-N S-SUnlike poles attract N-S

Magnetic Poles Are Not Charges

Single poles cannot be isolatedMagnetic Monopoles do not exist in natureBreak a magnet:Get two smaller ones

N S

S NN S

Ferromagnetic Materials

Show strong magnetic effectsIronCobaltNickelGadoliniumNeodymium

Permanent Magnets

Hi tech Neodymium iron boron magnets

Magnetic Field

Earth has fieldLines go from North to South

Units of Magnetic Field B

Tesla (SI Unit)Gauss (cgs unit)1 Tesla = 104 GaussEarth magnetic field about 0.5 gauss

Direction of Magnetic FieldThe direction the north pole of a compass would point when placed at that location

Ferromagnetism

Magnet made of domains1 mm lengthEach acts like tiny magnetNormally domain cancelExternal field aligns domainsStrong magnet can make other ferromagnetic materials into permanent magnets

Electrons Have Spin

Even permanent magnets owe strength to “currents”No way to divide a current and get N or S poleMagnetism is electrical in origin

Earth’s Magnetic Field

Very weakLike bar magnetNorth magnetic poleSouth magnetic pole

Electric Currents Produce Magnetism

Magnetic field around long straight wire

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Right hand rule determines direction of magnetic field

Right Hand Rule(s)

Long Straight Wire (Rule #1) Point thumb in direction of current Fingers wrapped around wire point in

direction of magnetic field

Circular loop of Wire (Rule #2) Curl fingers around wire with tips in

field direction Thumb points in direction of current

Alternate (preferred) version of Second RHR

Put curled fingers in current direction around loop or loops; thumb points in field direction INSIDE loop or coil.

Force on Current Carrying Wire

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F = BIL sin

is angle between

field and wire

Force is perpendicular to both current and field direction

Third Right Hand Rule

Long straight fingers in (positive) current direction (or direction of moving charged particle). Curled fingers in magnetic field direction, thumb points in direction of force on current carrying wire or positive charged particleIf particle is negative, change answer

Force on Moving Charged Particle in Uniform Magnetic Field

F = BqvsinThis force is perpendicular to the magnetic field and particle velocity vector

Charge Particle Path in Uniform Magnetic Field

Circle or helixF = maqvB = mv2/r (centripetal acceleration)r = mv/qBDirection follows right hand rule

How can F = BIL sinbe Used to measurea Field?

Hint: use a rectangular loop of wire

Force on a Charged particle in a Magnetic Field

Demo

F = qvB sinForce perpendicular to both particle direction and field

Magnetic Field Due to Straight Wire

B = 0I/2r

F = BIL0 permeability of free space

4 x 10-7

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Force Between Parallel Wires

F/l = (o/2 I1I2/L

Force per unit length of wireL is distance between wiresParallel currents attractAntiparallel currents repel

Electrical vs. Magnetic Forces

Similarities Both involve attracting and repelling Both decrease with distance

Differences Isolated poles do not exist Only electrical forces can be produced by

stationary charges Only moving charged particles experience

magnetic force Only electrical forces can do work

Magnetic forces on charged particles are perpendicular to field direction but electrical forces are in or opposite to electric field direction