Magnetism Force of Mystery Presentation Text ©2001 Philip M. Dauber as modified by R. McDermott

Magnetic Poles North and South Like poles repel N-N S-S Unlike poles attract N-S Poles are named for which way they point if suspended (so they rotate freely) A magnetic north points to geographic north

Magnetic Poles Are Not Charges A north pole is not a positive charge. A south pole is not a negative charge. Magnets do not exert forces on stationary charges. Charged objects are not affected by magnets.

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

Ferromagnetic Materials Show strong magnetic effects Iron Cobalt Gadolinium Neodymium

Ferromagnetism Ferromagnetic material contains “domains” 1 mm in length and normally random in direction Each acts like tiny magnet Generally, domains cancel – no magnetic effects An external field aligns domains (non-random) A strong magnetic field can make other ferromagnetic materials into permanent magnets

Electrons and Magnetism Magnetism is electrical in origin Magnetic fields are produced when charges move Even permanent magnets owe their strength to electron “currents” There is no way to “divide” a current to get N or S pole Stationary charges are unaffected by magnetic fields, and do not generate magnetic fields.

Permanent Magnets Hi tech Neodymium iron boron magnets

Units of Magnetic Field Tesla (SI Unit) 1 Tesla = 1 Weber/m 2 Earth’s magnetic field = 5x10 -5 Tesla

Earth’s Magnetic Field Very weak Like a bar magnet North magnetic pole is at the south geographic pole South magnetic pole is at the north geographic pole

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

Magnetic Field Magnetic field lines (flux) are measured in webers Magnetic lines of flux go from North to South

Electric Currents = Magnetism Magnetic field around long straight wire I Right hand rules determine the direction of magnetic fields Demo –Point thumb in direction of current –Fingers wrapped around wire point in direction of magnetic field

Electric Currents = Magnetism Magnetic field due to circular loops (solenoid) Demo – Grab loop with thumb of right hand in the direction of conventional current. – Fingers point in direction of field at center of loop.

Right Hand Rule(s) Long Straight Wire –Point right thumb in direction of conventional current –Fingers wrapped around wire point in direction of magnetic field Circular loop(s) of Wire –Grab loop with thumb in current direction –Fingers point in direction of field at center of loop

Force on a Charged particle in a Magnetic Field F = qvB sin  Magnetic force is perpendicular to both particle direction and field (vector cross-product) Demo

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

Force on Current-Carrying Wire I F = BIL sin   is angle between field and wire Force is perpendicular to both current and field directions

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

Magnetic Field Due to Straight Wire B =  0 I /(2  r) *  0  4  x10 -7 T-m/A I * Permeability of free space

Force Between Parallel Wires F/ L =  o I 1 I 2 /(2  d) Force per unit length of wire d is distance between wires Unidirectional currents attract Bidirectional currents repel

Induced EMF Moving a wire across magnetic field lines produces an EMF (potential difference) across the wire. This potential difference increases with increasing wire speed. The maximum EMF occurs when the wire is moved perpendicular to the field lines. = LvB sin 