An electric current that flows continuously in a single direction is called a direct current, or DC. The electrons in a wire carrying direct current move slowly, but eventually they travel from one end of the wire to the other because they keep plodding along in the same direction.
The voltage in a direct-current circuit must be constant, or at least relatively constant, to keep the current flowing in a single direction. Thus, the voltage provided by a flashlight battery remains steady at about 1.5 V.
The positive end of the battery is always positive relative to the negative end, and the negative end of the battery is always negative relative to the positive end. This constancy is what pushes the electrons in a single direction.
Another common type of current is called alternating current, abbreviated AC. In an alternating-current circuit, voltage periodically reverses itself. When the voltage reverses, so does the direction of the current flow.
In the most common form of alternating current, used in most power distribution systems throughout the world, the voltage reverses itself either 50 or 60 times per second, depending on the country. In the United States, the voltage is reversed 60 times per second.
Alternating current is used in nearly all the world's power distribution systems, for the simple reason that AC current is much more efficient when it's transmitted through wires over long distances. All electric currents lose power when they flow for long distances, but AC circuits lose much less power than DC circuits.
The electrons in an AC circuit don't really move along with the current flow. Instead, they sort of sit and wiggle back and forth. They move one direction for 1/60th of a second, and then turn around and go the other direction for 1/60th of a second. The net effect is that they don't really go anywhere.
For your further enlightenment, here are some additional interesting and useful facts concerning alternating current:
A popular toy called Newton's Cradle might help you understand how alternating current works. The toy consists of a series of metal balls hung by string from a frame, such that the balls are just touching each other in a straight line.
If you pull the ball on one end of the line away from the other balls and then release it, that ball swings back to the line of balls, hits the one on the end, and instantly propels the ball on the other end of the line away from the group.
This ball swings up for a bit, and then turns around and swings back down to strike the group from the other end, which then pushes the first ball away from the group. This alternating motion, back and forth, continues for an amazingly long time if the toy is carefully constructed.
Alternating current works in much the same way. The electrons initially move in one direction, but then reverse themselves and move in the other direction. The back and forth movement of the electrons in the circuit continues as long as the voltage continues to reverse itself.
The reversal of voltage in a typical alternating current circuit isn’t instantaneous. Instead, the voltage swings smoothly from one polarity to the other. Thus, the voltage in an AC circuit is always changing. It starts out at zero, then increases in the positive direction for a bit until it reaches its maximum positive voltage, and then it decreases until it gets back to zero.
At that point, it increases in the negative direction until it reaches its maximum negative voltage, at which time it decreases again until it gets back to zero. Then the whole cycle repeats itself.