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The electric current in a copper wire consists of electrons that are moving in the same direction. In the case of direct current, the direction is constant. The wire can extend for miles, but the instant that the electricity is turned on, it seems to reach the full length of the wire.

At first glance, it would appear that electrons travel with the speed of light. However, when we consider that the electrons start from rest, and must spend some time accelerating, they must fall short of maximum speed.

Furthermore, if we concentrate on the electron that is closest to the switch, we find that its motion is opposed by zillions of electrons that block its path. It is lucky to be moving at all. When the switch is turned on, the nearest electron has more electrons per cubic centimeter behind it than in front of it. Thus the electron is pushed harder from behind than from ahead. It starts at zero speed and reaches a speed that is determined by the ability of the power supply to add new electrons at one end of the wire and remove electrons at the other end.

The mystery is how the power arrives instantly at the end of miles of wire when the switch is turned on at the power station. The answer is that the signal, not the actual electron, travels the length of the wire at close to the speed of light.

As I have written in my essay, The Building Blocks Of Matter, electrons have a core of bits of negative charge, which is surrounded by a shell of neg-pos pairs. The paired bits are oriented with the pos toward the core, and the neg away from the core.

A resting electron receives equal numbers of neg-pos pairs from all directions in a steady downpour. The electron emits neg-pos pairs in all directions at the same rate as it receives them. When a neighboring electron approaches, the neg-pos that is between the two electrons becomes more dense. The resting electron is repelled by the negative ends of the neg-pos pairs, and starts to move. The moving electron is repelled by the negative ends of the neg-pos pairs that were sent by the resting electron. It therefore loses some of its speed.

The signal we spoke of is the neg-pos pair. It moves at the speed of light for the very good reason that the light itself is composed of neg-pos pairs that are in flight.

When one electron transfers motion to another electron it loses some of its own motion. The density of neg-pos between the interacting electrons pushes both ways, accelerating one and decelerating the other. In order for the current to be maintained, the power supply must send in a new electron for each displaced electron.

While we were solving one mystery, we solved two. The second mystery was how a body gains mass as it gains speed. The extra neg-pos that an electron gains, it keeps as long as it continues moving, and loses when it slows or stops.

The same wire that carries a particular current can, at other times, carry a larger or a smaller current, depending on the ability of the power source to replenish the supply of electrons.Of course the power supply also has to collect electrons that arrive at the other end of the wire. For a given power source, the power can be altered in several ways. One could shorten the wire and find that the current increases. Another way to is to use two wires in parallel. But each wire carries the same current as before.

The ability to carry a current is called conductance. Copper is a better conductor than steel. The reason can be found in the details of the copper atoms as compared with those of iron atoms. The positive charge of the atom is equal to the number of protons in the nucleus. The net positive charge of the nucleus determines how strongly the electrons will be held. The outermost electron is the one that is active in an electric current. Beside the attractive force of the nucleus, the repulsive force of the other electrons affect the motion of the active electron. The point is that the behavior of the active electron is complicated and involves forces that retard the electron and make it a less perfect conductor. Just an instant after the switch is turned to the off position, the electron stops flowing.

We expect a body in motion to remain in motion, but the electrons stop moving as soon as they stop being pushed. The parts of the copper atom that are not moving with the current are caused to move by the interaction with the moving electrons. The force between interacting particles acts equally in both directions. For a given force, the particle with more mass moves slower than the particle with less mass. While the electron moves fast enough to escape from an atom, the atomic nucleus moves too slowly to change its position in the wire. However the nucleus of the copper atom does vibrate. When the current stops, the motion that is lost by the electron is gained by the atomic nuclei and some of the electrons. These parts move back and forth like a pendulum. The intensity of the motion can be measured with a thermometer.

The degree to which a wire falls short of perfect conductance is called resistance. If a foot of wire is cut in half, each half has half the resistance. If the two halves are placed side by side, and become joint conductors, their resistance becomes one fourth the resistance of a foot of wire. In the same manner, A wire whose cross section area is twice as much as the cross section area of another wire has half the resistance.

Current in a wire is measured by the number of electrons that pass a point in one second. If the current is the same for a narrow wire and a thick wire, the temperature in the thick wire will be lower than the temperature of the narrow wire. Since the narrow wire has fewer electrons per cross section, the electrons in that wire have to move faster to have the same number pass a point in one second. The speed of these electrons is reflected in the increase in vibration of the copper atoms. Carried to the extreme, the vibration can cause the copper to melt or evaporate.

One can take a single strand of wire two inches long and connect its ends to the ends of a lampcord. When the other end of the cord is plugged into a receptacle, the single strand will explode spectacularly.

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