Electricity and Magnetism

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Transcript Electricity and Magnetism

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 move around the nucleus of an
 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,
 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
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 such
a neat
set of
 Since moving charges cause magnetism, an
electric current will produce a magnetic
 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
 The source of almost all our electricity
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
 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
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
 With AC and transformers, voltage can be
increased and current decreased for
 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
 Used in cars and Spencer Gift discharge