Magnetism

Magnetism - 1

December 03, 2018

Magnetism!

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

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

Also:

- Like poles repel
- Opposite poles attract
- Each magnet must have at least one North and one South pole (though they may have more than one of each). There is NO such thing as a magnetic monopole.
- Magnetic fields are real, but the lines are imaginary - Field lines indicate the direction that a compass needle would take in the vicinity of the magnetic field.
- There are naturally occurring magnetic minerals - a very common one is called magnetite (Fe3 O4)

Magnetic north on the Earth is near Ellesmere Island in Northern Canada, several hundred miles from true (geographic) North (the North Pole). It is moving toward Russia at several miles per year.

For gory detail:

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

To find True/Geographic north, it is easiest to find Polaris (the current north star). Polaris is actually not all that bright, though in the top 50 brightest stars in the night sky. You need to find the Big Dipper (asterism at the rear end of Ursa Major). Follow the “pointer stars” at the end of the dipper. These visually lead you to Polaris. [If you were to follow the “arc” of the handle, you’d come to a bright star, Arcturus – “Follow the arc to Arcturus.”]

FYI:

https://www.youtube.com/watch?v=Ws6AAhTw7RA

The quantum levitation video shown in class.

http://skymaps.com/

How do we get magnetism?

Magnetic fields are related to electrons spins. Electrons act like tiny magnetic spinning tops. There is a tiny magnetic element associated with each electron spin. If the spins align, more or less, the object is said to be somewhat magnetic. More spin alignments (domains) means more magnetism. Materials that do this easily are generally said to be ferromagnetic.

As it happens, metals do this best (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 traveling through a wire, electrons can be thought of as moving together – this causes a magnetic field, also known as electromagnetism. The magnetic field caused 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 it) can produced quite a strong electromagnet.

A coil with current running through it can naturally react to a permanent magnet – if this is engineered well, we have a motor. See illustrations and demos in class.

Electromagnetic Induction

Current causes magnetism – something shown in the early 19^th century by Hans Oersted. As it happens, the reverse is also true – magnetism can cause current, but there must be some relative CHANGE in the magnetic field or location of conductor. There must be relative change – either coil or magnet must move, relative to the other.

This phenomenon, wherein a change in magnetic field relative to a conductor, generates electric current is called “electromagnetic induction.” It is the secret to understanding generators. If something, say moving water from Niagara Falls, can cause a coil of wire (in a turbine) to spin, current is generated. More spins of wire means more current.

It’s all about moving conductors in magnetic fields

In conclusion:

Electromagnetism:

Current (moving charges) creates a Magnetic Field

Electromagnetic Induction:

Change in magnetic field (through conductor), or vice versa, creates an electric current

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Some images related to the above:

Magnetic field lines shown with iron filings and compasses: