Magnetism and Induction

Early Ideas

Described by Ancient Greeks “Lodestones” found on the Isle of Magnesia North and South poles Magnets obey Abdul’s Principle No single poles!

Permanent Magnets

Magnet fields are generated by moving electrons.

Electrons move around the nucleus of an atom.

Therefore all atoms have magnetic fields. In most materials these fields are random. In magnetic materials the fields line up. Called “magnetic domains”.

How to make a magnet

Choose a ferromagnetic material (iron, steel)

Stroke it with a magnet in one direction, removing the magnet after each stroke.

Heat or vibration speeds up the process. But they can also demagnetize a temporary

magnet!

Magnetic Field Lines

Leave the North pole and circulate to the South pole.

Much like electric field lines.

Magnetic Materials

Lodestone (magnetite mineral) Neodymium: very strong! Alnico: common magnetic alloy

Earth as a magnet

The core of the Earth is solid/molten iron Loosely coupled to the rotation of the crust,

like an auto’s clutch slipping. The circulating iron’s free electrons create

the fields. South pole of this magnet actually off

Greenland (still called Magnetic North). The poles flip about every 500,000 years

Not sucha neatset of fieldlines!

Electromagnets

Since moving charges cause magnetism, an electric current will produce a magnetic field.

The field is just like that of a bar magnet. Can make a very strong magnet with only

one D cell. And electromagnets can be turned off!

Michael Faraday

1830’s: he (and Joseph Henry in the US) discovered that a changing magnetic flow will produce an electric current in a wire.

The key is a changing flow; either a change in the area of a loop of wire or in the field itself.

The source of almost all our electricity today!

Electromagnetic Induction

Faraday’s Law changed the world within twenty years;

Morse’s telegraph shrunk the world via instant communication.

By the turn of the century electricity produced by induction powered light, trolleys, and factories.

Motors and Generators

Essentially the same apparatus. Turn a coil of wire in a magnetic field and

you produce current. Run a current through a coil in a magnetic

field and you will make it turn.

Motor or Generator

Transformers

More than meets the eye Transformers have the capability to change

AC voltage--won’t work with DC. Transformers have two coils. In one a current

changes with time. This induces a changing voltage in the other coil.

The induced voltage is higher or lower depending on the number of loops in the coil.

Why we use AC

Since DC can’t be transformed, sending enough power down a wire to light a town would melt the wire.

The heat produced by a current carrying wire is proportional to the square of the current.

With AC and transformers, voltage can be increased and current decreased for transmission.

Tesla

Not the 80’s band from Sacramento Nichola Tesla was an eccentric scientist at the

turn of the last century. He invented AC, funded by Westinghouse. He also invented the Tesla coil, essentially a

transformer. Used in cars and Spencer Gift discharge

globes.