by Earl King

Created on: July 12, 2008

ALTERNATING CURRENT (AC) AND
DIRECT CURRENT (DC):
A SIMPLE EXPLANATION

The simplest explanation would be dc current flows in one direction. Alternating current flows first one way then the other. But this really doesn't teach us anything. If we are going to spend time looking at how this comes about. I think that will still be a simple explanation.

Let's start with direct current (dc) since it is the simplest. This is the current supplied by batteries. Take a storage battery like the one that starts your car. It has two terminals, one positive and one negative. There is an excess of electrons (carries a negative charge) at the negative terminal. There is a lack of electrons at the positive terminal. Electric current, of course is a flow of electrons. So if we connect the two terminals with a wire or cable, there will be a flow of electrons from the negative terminal through the wire to the positive terminal. Never hook it up that way however. The wire, being of very low resistance, will heat up. Run the current through a load, such as a heater, or coil anything that consumes power. These things have resistance and will limit the magnitude of the current.

Close the switch and the current will increase to a maximum that is dependent upon the voltage of the battery, the internal resistance of the battery and the resistance of the wire and whatever load it supplies. It will flow steadily in one direction, from the negative pole to the positive pole. As the battery charge is reduced, the current decreases, but always in the same direction.

Moving on to alternating current (ac), we find this is a little more complex. As the name implies, the current (same flow of electrons) flows first in one direction, building up to a maximum. It then decreases to zero and still flowing in the same direction. At zero the direction of the current reverses, flowing to a maximum in the opposite direction then decreases again to zero.

WHAT MAKES IT ALTERNATE AND WHAT IS SO GREAT ABOUT THAT?
To go further in our understanding of the alternating current, we must take a look at magnetism. The Danish scientist Oersted found the connection between electricity and magnetism. Consider a bar magnet with the north pole on the left and the south pole on the right. The poles were so named because if you suspended the bar, the north pole would swing roughly toward earth's north. Taking a small compass and moving it around the bar magnet, we see where the needle points in different directions depending

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