Magnetism What makes some materials magnetic? Iron Nickel Cobalt Where do we see magnets?

Magnets Compass Electromagnets at junkyards Meters (ammeter, voltmeter) Motors, Generators Loudspeakers Magnetic Tapes (sound and video) Maglev Trains Superconducting Magnets

Magnets Bar Magnets Objects attracted to ENDS (poles) NORTH SOUTH Like poles repel, unlike poles attract

Comparison to Electric Charges Magnetic poles cannot be isolated like electric positive and negative charges If you cut a magnet in half, you will still have two poles, North and South, on each piece

So why are some materials magnetic? Electrons spin on axis like a top A change in motion of a charge produces a magnetic field (we will learn more about this later…) Atoms have many electrons, so usually the electron spins cancel out

Ferromagnetic Materials In SOME materials (Fe, Ni, Co), the fields do not cancel out completely These are called “Ferromagnetic”

Magnetic Domains Large groups of atoms whose net spins are aligned are called “magnetic domains” 10-4 to 10-1 cm across In unmagnetised substances, the domains are random When placed in a magnetic field, the domains get more aligned

Aligning Magnetic Domains You can make a magnet out of an unmagnitized piece of iron. Rub with magnet Put near big magnet

Types of Magnets

Magnetic Fields When field lines are close together, the magnetic field is stronger

Using Iron Filings to See Magnetic Field

Strength of Magnetic Field The symbol for magnetic field strength is B This is a vector quantity (has magnitude and direction) The units for B are the Gauss or Tesla 10,000 Gauss = 1 Tesla The Earth’s magnetic field B = 1 Gauss

Cosmic Rays are Deflected by the Earth’s Magnetic Field

Aurora Borealis