From the Greek meaning 'heavy with wine'
A blog devoted to science and reason
Written after a glass or two of Pinot Noir.

Friday, September 2, 2011

Maxwell's Silver Magnet (apologies to the Beatles)

In the post on wine swirling, i didn't bring up one of of Ralph de Amicis' incorrect comments, because there were so many that were wrong. This one though, deserves it's own post.

The positive pole is more highly charged, just like the North Pole of the Earth,…

A frequent mistake that people make is to mix up the electric charge with the magnetic pole. Both are basic characteristics of matter, just as mass is, but they're not interchangeable. Incredibly, I've even heard one of my science colleagues make this mistake in his class.

When I rub a balloon on my sweater, the balloon rips electrons from the fibers. When I put the balloon near the wall, those extra electrons on the balloon repel the electrons near the surface of the wall leaving a slight deficit of electrons (or a slight surplus of protons). Now notice those little plus (+) signs and negative (-) signs. Benjamin Franklin realized that electric charge comes in two types. He named them positive and negative. I think those are rather apt names, because in mathematics a negative sign can mean 'opposite.'

Now let's play with a magnet. When I place a magnet near a compass which is just a small magnet, I notice that the one end of the compass (white in the figure) is attracted to the red end of the magnet and vice-versa. I also notice that the red end of the compass is repelled by the red end of the magnet. When I take the compass outside, I see that the red end always points North. Historical note: The Chinese invented the compass 4500 years ago.

Since not all magnets have red and white ends, we need to name the two ends. I guess that since the red end points toward the North Pole, some ancient people (probably either the Chinese or the Greeks) called that end the north pole and the other end the south pole. Now look at the figure with the Earth. Since the north pole of the compass (the red end) points toward the North Pole of the Earth, the North Pole is really a magnetic south pole. Every semester when I teach this, I wish we used the terms red end and white end.

Two separate characteristics, two different phenomena. Charges created static electricity. Poles create magnetism.

What's really cool though is that although charge and pole are different characteristics of matter, they are related. Scientists like Faraday and Maxwell realized in the 19th century that by moving charges (like in electrical currents), we can create an electromagnet and by moving magnets we can create electrical current. These are the principles that give us electrical motors and generators.

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