CURRENT FLOW

If a conducting or semiconducting path is provided between two poles having a poten- tial difference, charge carriers will flow in an attempt to equalize the charge between the poles. This flow of electric current will continue as long as the path is provided, and as long as there is a charge difference between the poles. Sometimes the charge difference is equalized after a short while. This is the case, for example, when you touch a radiator after shuffling around on the carpet in your hard-soled shoes. It is also true in a lightning stroke. In these instances, the charge is equalized in a fraction of a second. The charge might take longer to be used up. This will happen if you short-circuit a dry cell. Within a few minutes, or maybe up to an hour, the cell will “run out of juice” if you put a wire between the positive and negative terminals. If you put a bulb across the cell, say with a flashlight, it takes an hour or two for the charge difference to drop to zero.
24 Electrical units
2-1 Electric lines of flux always exist near poles of electric charge.
In household electric circuits, the charge difference will essentially never equalize, unless there’s a power failure. Of course, if you short-circuit an outlet (don’t!), the fuse or breaker will blow or trip, and the charge difference will immediately drop to zero. But if you put a 100-watt bulb at the outlet, the charge difference will be maintained as the current flows. The power plant can keep a potential difference across a lot of light bulbs indefinitely. You might have heard that “It’s the current, not the voltage, that kills,” concerning the danger in an electric circuit. This is a literal truth, but it plays on semantics. It’s like saying “It’s the heat, not the fire, that burns you.” Naturally! But there can only be a deadly current if there is enough voltage to drive it through your body. You don’t have to worry when handling flashlight cells, but you’d better be extremely careful around household utility circuits. A voltage of 1.2 to 1.7 V can’t normally pump a dangerous cur- rent through you, but a voltage of 117 V almost always can. Through an electric circuit with constant conductivity, the current is directly propor- tional to the applied voltage. That is, if you double the voltage, you double the current; if the voltage is cut in half, the current is cut in half too. Figure 2-2 shows this relationship as a graph in general terms. But it assumes that the power supply can provide the neces- sary number of charge carriers. This rule holds only within reasonable limits.