"Opposite charges attract; like charges repel" is the basic truth about electricity. And because of this we are able to use the most mobile of charges, the electron, to perform a great deal of work.
Electrons and Shells
Electrons cluster about a proton-packed nucleus until negative and positive charges exactly match, as many electrons as protons. In fact, electrons are constrained to fill discrete energy "slots" when attracted to the neighborhood of a nucleus. The first, innermost set of slots can accommodate up to two electrons, the next eight, the next eighteen . . . the rules are complicated, but starting from closest to the nucleus each set of slots, called a shell, fills up before an electron must be kicked upstairs to the next shell.
If, for example, the third shell is full (for a total of 28 electrons), but the nucleus contains 29 protons, a twenty-ninth electron will be attracted to this atomic configuration but will have to occupy a lonely outlying slot, becoming the only resident of the fourth shell. The element that fits this description happens to be copper, and its electron arrangement has some important implications for ordinary electricity: the isolated outer electron has difficulty "seeing" a positive charge at the copper atom's center. The completed shell beneath it, an evenly distributed swarm of electrons, masks the last proton's attraction. That twenty-ninth electron is, thus, loosely attached and easily dislodged.
The ways in which electrons try to fill shells, sometimes sharing slots in two atoms at once (forming molecules), or flying off altogether (leaving ions behind) is the substance of chemistry and of life. This complexity, from table salt to DNA, is all about electrical charge. So is the everyday power that we tap into at a wall outlet.
A Closer Look at Charge
Whether positive or negative, every charged particle carries a magnetic field. Charge and field are inseparable aspects of the same force, the electromagnetic force. The two quantities interact constantly. Every electron (or proton, for that matter) is a magnet, with its own north and south pole. There are some more rules, like the shell rules, that describe how these tiny magnets may align themselves in an atom, canceling out, so that atoms are not left noticeably magnetic.
If, however, some electrons are forced to move all in the same direction along a conductor, like a copper wire, they build up a magnetic field around the wire. Conversely, if a magnetic field is passed across the wire, it will cause electrons to movea phenomenon termed induction.
Of course, energy of some kind must be applied to pry electrons away from the protons they would prefer to orbit. Batteries separate charges by chemical means, as do the mitochondria in living cells, our own tiny batteries. Dragging the rubber sole of a shoe across synthetic or wool carpet forces charges apart by physical means, friction in this case. Dynamos and generators use magnetic fields to push electrons into motion through induction.
And wherever an excess of charge is built up, it will flowif given the opportunitytoward some point with less charge. The difference in charge between one point and another is voltage, commonly stated in volts. The higher the voltage, the more force acting to move charges from one place to another. The number of electrons flowing, the strength of the current, is measured in amperes, or just amps, for short. The path a current is made to take, presumably to accomplish some useful work, is called a circuit. In a modest electrical device, say a flashlight, the voltage and current are not large, and the circuit is quite simplebut it took a few centuries of scientific inquiry to arrive at a comfortable grasp of the principles involved.