Science Weblecture for Unit 31
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As you may remember from our early discussion of "force", scientists now identify four major forces: gravity, electromagnetic force, and the strong and weak nuclear forces that hold the nuclei of atoms together. In our daily lives, we normally only encounter the first two forces in ways we can easily see. Gravity holds us to the surface of the earth. Static electricity shocks us when we touch a metal doorknob on a dry day and current electricity runs our electronics, making it possible for you to read this web page. Magnetism swings round the compass that guides an airplane to its landing field, generates electricity from the spinning wheels on your car, and holds your little brother's latest artistic creation, or that note to remember to mow the lawn, to the refrigerator door. We experience the practical results of attractive and repulsive magnetic forces every day.
If you've ever played with Brio trains or similar brands that use magnets to connect the train cars, you know that you have to get the "correct" ends of the train cars together, or they will move apart instead of sticking.
All magnets work this way: they have two opposite centers of force, called poles. We could, as we do with electrical charges, just name these opposing centers "plus" and "minus", but because the earth itself acts like a magnet, we've come to name them after the direction a magnetized compass needle points (more about that later).
Strictly speaking, an attractive force always reduces the distance between two objects, and a repulsive force always increases this distance, so one way to determine whether a force is attractive or repulsive is to identify a source of the force, and then put another source of the force nearby, and see what happens. Unlike an electrically charged particle, which will have a net all negative or all positive charge, there are no magnetic monopoles. All magnets have two poles, one "north" and one "south". If we put the two north poles of magnets together, they push apart. If we put the two south poles of two magnets together, they push apart. Only if we put a north pole on one magnet and a south pole on the other magnet together will they attract each other and "stick". This leads to the rule:
Opposite magnetic poles attract one another, like magnetic poles repulse each other.
We can divide matter into three types according to their magnetic properties:
Materials like glass and plastic are also not affected by nearby magnets: they don't move in response to a magnet's presence, and they don't attract or repel magnets. This leads to another observation: magnets only affect other magnets. Since magnets can change the path of a moving electrical charge, charged particles when they are moving are also magnets. We'll come back to this when we talk about Faraday's discovery of the relationships between electricity and magnetism.
Gilbert's greatest discovery was that the Earth itself was a magnet. Having the earth as a magnet has been a great advantage, since we have a common magnetic field (well, mostly) affecting the behavior of small magnets like compass needles. A magnetized object that is free to move will spin until one end (the "north" end) points toward the Earth's magnetic pole in the northern hemisphere. Of course, the "opposites attract" rule means that the magnetic pole in the earth's northern hemisphere, the one that attracts the magnetic north of the compass, must be a south magnetic pole!
Because the earth's magnetic pole is not at the geographic north pole, mapmakers and orienteers (people who navigate around by using compasses and measuring distances) must make a correction when they take a compass reading.
Just to make things more interesting, the earth's magnetic pole doesn't stay put.
Read about The Earth's Inconstant Magnetic Field at the NASA Science site.
You may remember that Aristotle required the agent applying the force to touch the object it wanted to move. Magnetism is an example of a force that causes action at a distance: one magnet doesn't have to be touching the like pole of a second magnet before the two will jump apart. If you try to push like poles of two magnets together, you will feel the repulsive force get stronger as you bring the magnets closer together.
This relationship between distance and force is called an inverse proportion law. Inverse means that as one quantity goes up, the dependent quantity goes down. With magnetic force (and electricity and gravity), the shorter the distant between the source of the force and the object being moved, the greater the force, so force goes up as distance goes down.
One way to envision how magnets work is to think of the space around them as a field through which lines of force can pass.
Any magnetized object that you put in the field will rotate until it is aligned with the field, then start to move along the field lines toward one of the poles. We can draw the field lines closer together to represent greater field strength. Notice that near the poles, the shape of the field already brings the lines together, so the magnetic force is strongest there.
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