Current Flow

If a conducting or semiconducting path is provided between two poles having a potential difference, charge carriers flow in an attempt to equalize the charge between the poles. This flow of current continues 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 while wearing hard-soled shoes. It is also true in a lightning stroke. In these instances, the charge is equalized in a fraction of a second. In other cases, the charge takes longer to be used up. This happens if you short-circuit a dry cell. Within a few minutes, the cell “runs 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.

In household electric circuits, the charge difference is never equalized, 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.

Have you heard that it is current, not voltage, that kills? 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 current through you, but a voltage of 117 V almost always can.

In an electric circuit that always conducts equally well, the current is directly proportional to the applied voltage. If you double the voltage, you double the current. If the voltage is cut in half, the current is cut in half too. Following Figure shows this relationship as a graph in general terms. It assumes that the power supply can provide the necessary number of charge carriers.

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Relative current as a function of relative voltage for low, medium, and high resistances.