Magnetism Force of Mystery demo

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 South Like poles repel N-N S-S Unlike poles attract N-S

Magnetic Poles Are Not Charges Single poles cannot be isolated Magnetic Monopoles do not exist in nature Break a magnet: Get two smaller ones N S S NNS

Ferromagnetic Materials Show strong magnetic effects Iron Cobalt Nickel Gadolinium Neodymium

Permanent Magnets Hi tech Neodymium iron boron magnets

Magnetic Field Earth has field Lines go from North to South

Units of Magnetic Field B Tesla (SI Unit) Gauss (cgs unit) 1 Tesla = 10 4 Gauss Earth magnetic field about 0.5 gauss

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

Ferromagnetism Magnet made of domains 1 mm length Each acts like tiny magnet Normally domain cancel External field aligns domains Strong 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 pole Magnetism is electrical in origin

Earth’s Magnetic Field Very weak Like bar magnet North magnetic pole South magnetic pole

Electric Currents Produce Magnetism Magnetic field around long straight wire I 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 I 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 particle If particle is negative, change answer

Force on Moving Charged Particle in Uniform Magnetic Field F = Bqvsin  This force is perpendicular to the magnetic field and particle velocity vector

Charge Particle Path in Uniform Magnetic Field Circle or helix F = ma qvB = mv 2 /r (centripetal acceleration) r = mv/qB Direction follows right hand rule

How can F = BIL sin  be Used to measure  a Field? Hint: use a rectangular loop of wire

Force on a Charged particle in a Magnetic Field Demo F = qvB sin  Force perpendicular to both particle direction and field

Magnetic Field Due to Straight Wire B =  0 I/2  r F = BIL  0 permeability of free space 4  x I

Force Between Parallel Wires F/l = (  o /2  I 1 I 2 /L Force per unit length of wire L is distance between wires Parallel currents attract Antiparallel 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