Magnetism

Some ideas from the Magnetism sessions

Similar to the case of charge, magnetic poles are divided into North and South poles.

A North magnetic pole is one that is attracted to the Earth's magnetic north pole. This means that the Earth's magnetic north is ACTUALLY A SOUTH POLE (magnetically speaking).

Like poles repel.
Opposite poles attract.
Each magnet MUST have at least one north and one south pole.
Magnetic fields are real, though the field lines are imaginary. Field lines indicate the direction that a compass needle would take in the vicinity of the magnet.

Magnetic north on the Earth is near Ellesmere Island in Northern Canada, several hundred miles from true (geographic) North - the North Pole.

For gory detail:

http://en.wikipedia.org/wiki/North_Magnetic_Pole

To find true/geographic North, it has historically been done by following Polaris (the pole-star, the lodestar, the North star). Polaris is actually not all that bright (though in the top 50). You need to find the Big Dipper (the rear end of Ursa Major) and follow the two pointer stars at the end of the scoop - these point to Polaris (which is in Ursa Minor). [If you were to follow the "arc" of the handle, it would take you to the bright star Arcturus, as in "follow the arc to Arcturus".]

FYI - "Star Hopping" is a great way to learn your way around the sky. I like the free star chart site:

http://www.skymaps.com/

Magnetic fields are related to electron spins. Electrons act like miniature (extremely miniature) spinning tops. There is a magnetic element associated with their spins. If spins align, more or less, an object can be said to be somewhat magnetic. More spin alignments (domains) means more magnetism. Materials that do this well are said to be ferromagnetic.

As it turns out, metals do this best, as they often have free electrons. In the core of the Earth, molten metal convects (rises and falls) giving the Earth a good magnetic field - measurable from the surface and beyond. Several planets have magnetic fields.

In general, the motion of charges leads to magnetic fields. If you have charge travel through a wire, electrons can be thought of as moving together - this causes a magnetic field. The magnetic field generated by a current passing through a wire is often small, but if you coil the wire upon itself, the magnetic fields add up. Several hundred turns of wire (with current running through them) can produce quite a strong electromagnet. Understanding electromagnets allows us to understand motors.

Current causes magnetism - something shown in the earth 19th century by Hans Oersted. As it happens, the reverse is also true: magnetism can causes current, but it must be a CHANGE in the magnetic field. The magnet (or conductor/coil through which it travels) must move. There must be some relative change between coil and magnet - either the coil must move or the magnet must move.

This is referred to as electromagnetic induction, and it is the secret to understanding generators. If a coil moves in a magnetic field, a current is generated inside it. Imagine turbines at the bottom of Niagara Falls (or any waterfall) - water hits the turbine and makes it spin. Inside the turbines are large coils of wire, free to rotate. These coils spin within a permanent magnetic field inside the turbine - this generate large amounts of current. Similarly, you can burn fossil fuels to heat water and generate steam. The steam is fed into a turbine, also generating large amounts of current.

It's all about moving conductors in magnetic fields!