Magnetism Notes

1. Magnetism Notes CP Physics Ms. Morrison

2. 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)

3. 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

4. Magnetic Poles vs. Electric Charges

5. 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

6. 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

7. 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

8. 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

9. 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

10. Magnetic Domains, pg 3

11. 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

12. 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)

13. 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)

14. 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

15. 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

16. 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

17. 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)