Magnetism Notes CP Physics Ms. Morrison

General Information Greeks discovered magnetic rocks called lodestones more than 2000 years ago Chinese used lodestones to help navigate ships Lodestones contain iron ore (magnetite)

Magnetic Poles Regions of the magnet that produce magnetic force Pole pointing northward – North Pole pointing southward – South Breaking a magnet in half will not separate the poles – only makes two smaller magnets

Magnetic Poles vs. Electric Charges Property Magnetic Poles Electric Charges likes repel, opposites attract √ force acts over a distance can separate charges (poles) from each other NO

Magnetic Fields Space around a magnet through which magnetic force is exerted, can be seen with iron filings (shows magnetic field lines) Magnetic field lines move from north to south pole More lines = stronger field Field strongest at poles

Magnetic Fields, pg 2 Moving charges create magnetic fields, ex. Spinning electrons Electrons = tiny magnets Pair spin in same direction = stronger magnet Pair spin in opposite direction = no magnet because their magnetic fields cancel out

Magnetic Fields, pg 3 Most materials NOT magnetic because electron pairs cancel out their magnetic fields Iron, nickel, and cobalt – have atoms with unpaired electrons whose magnetic fields are not entirely canceled out – so have magnetic properties Iron has four unpaired electrons so each iron atom is a tiny magnet

Magnetic Domains Large clusters of aligned atoms – their magnetic fields are so strong that they line up with each other Magnets have domains that are lined up while ordinary iron and other materials do NOT have aligned domains

Magnetic Domains, pg 2 Can create permanent iron magnets by placing the iron in a strong magnetic field Can create temporary magnets by placing materials with iron in them near a strong magnetic field, once field gone domains will revert to original state Can destroy permanent magnets by dropping them or heating them up – causes domains to be jostled out of alignment

Magnetic Domains, pg 3

Electric Current and Magnetic Fields Moving charges produce magnetic fields Current through a wire creates a magnetic field around the wire – Oersted (1820) Reversing current flow reverses direction of the magnetic field

Electromagnets A current-carrying coil of wire with many loops Has a north pole and a south pole Strength of electromagnetic affected by three factors # of loops in the coil (more loops = stronger) Amount of current (larger current = stronger) Presence of iron core (present = stronger)

Magnetic Forces and Charged Particles Charged particles at rest are not affected by magnetic field (stationary) When moving, charged particles are deflected by magnetic fields Used in TV tubes to create picture on screen Earth’s magnetic field deflects charged particles from outer space (cosmic rays)

Magnetic Forces and Current-Carrying Wires Current-carrying wire placed in magnetic field is deflected Reversing current cause deflection in opposite direction Greatest deflection occurs when current is perpendicular to magnetic field

Meters Meters use magnetic needle Current moving through wires in meter creates magnetic field that interacts with magnetic field of needle Cause needle to be deflected (like repel, etc.) Amount of deflection indicates amount of current

Motors Motors Electric current through coil of wire inside a permanent magnet Coil of wire creates magnetic field that interacts with magnet and begins spinning Current must reverse every half turn so that two magnetic fields keep repelling each other So electrical energy is converted into mechanical energy

Earth’s Magnetic Field Earth is huge magnet (magnetic poles not same as geographic poles) Thought to result from moving charges within molten part of Earth beneath crust NOT stable – wanders, diminishes to zero and then reverses itself Holds Van Allen Radiation belts around Earth (aurora borealis)