2. Magnetism and Induction

3. Early Ideas 4 Described by Ancient Greeks 4 “Lodestones” found on the Isle of Magnesia 4 North and South poles 4 Magnets obey Abdul’s Principle 4 No single poles!

4. Permanent Magnets 4 Magnet fields are generated by moving electrons. 4 Electrons move around the nucleus of an atom. 4 Therefore all atoms have magnetic fields. 4 In most materials these fields are random. 4 In magnetic materials the fields line up. 4 Called “magnetic domains”.

5. How to make a magnet 4 Choose a ferromagnetic material (iron, steel) 4 Stroke it with a magnet in one direction, removing the magnet after each stroke. 4 Heat or vibration speeds up the process. 4 But they can also demagnetize a temporary magnet!

6. Magnetic Field Lines 4 Leave the North pole and circulate to the South pole. 4 Much like electric field lines.

7. Magnetic Materials 4 Lodestone (magnetite mineral) 4 Neodymium: very strong! 4 Alnico: common magnetic alloy

8. Earth as a magnet 4 The core of the Earth is solid/molten iron 4 Loosely coupled to the rotation of the crust, like an auto’s clutch slipping. 4 The circulating iron’s free electrons create the fields. 4 South pole of this magnet actually off Greenland (still called Magnetic North). 4 The poles flip about every 500,000 years

9. Not such a neat set of field lines!

10. Electromagnets 4 Since moving charges cause magnetism, an electric current will produce a magnetic field. 4 The field is just like that of a bar magnet. 4 Can make a very strong magnet with only one D cell. 4 And electromagnets can be turned off!

11. Michael Faraday 4 1830’s: he (and Joseph Henry in the US) discovered that a changing magnetic flow will produce an electric current in a wire. 4 The key is a changing flow; either a change in the area of a loop of wire or in the field itself. 4 The source of almost all our electricity today!

12. Electromagnetic Induction 4 Faraday’s Law changed the world within twenty years; 4 Morse’s telegraph shrunk the world via instant communication. 4 By the turn of the century electricity produced by induction powered light, trolleys, and factories.

13. Motors and Generators 4 Essentially the same apparatus. 4 Turn a coil of wire in a magnetic field and you produce current. 4 Run a current through a coil in a magnetic field and you will make it turn.

14. Motor or Generator

15. Transformers 4 More than meets the eye 4 Transformers have the capability to change AC voltage--won’t work with DC. 4 Transformers have two coils. In one a current changes with time. This induces a changing voltage in the other coil. 4 The induced voltage is higher or lower depending on the number of loops in the coil.

16. Why we use AC 4 Since DC can’t be transformed, sending enough power down a wire to light a town would melt the wire. 4 The heat produced by a current carrying wire is proportional to the square of the current. 4 With AC and transformers, voltage can be increased and current decreased for transmission.

17. Tesla 4 Not the 80’s band from Sacramento 4 Nichola Tesla was an eccentric scientist at the turn of the last century. 4 He invented AC, funded by Westinghouse. 4 He also invented the Tesla coil, essentially a transformer. 4 Used in cars and Spencer Gift discharge globes.