513. How do power lines work and what is the purpose of all the electrical things you see behind the fences with signs saying "Warning: High Voltage"?
Electric power is distributed over long distance using high voltages and relatively low currents. Since the amount of power that flows through a wire is equal to the product of its voltage (the amount of energy carried by each unit of electric charge) and its current (the number of units of electric charge that flow through the wire each second), the electric company can distribute its power either as a large current at low voltages or a small current at high voltages. But it turns out that the amount of power that's wasted by electricity as it flows through a wire is proportional to the square of the current in that wire. Thus the more current that flows through a wire, the more power that wire turns into thermal energy (or heat). To minimize this energy loss, the power company uses transformers to convert the electricity to small currents at very high voltages for transmission cross country. Near each community, there is then a power substation at which this very high voltage power is converted to lower voltage forms. Even in neighborhoods, they use medium currents at moderately high voltages to avoid power wastage. Only in the vicinity of your home is the electricity finally converted by transformers to a large current at low voltage for safe delivery to your appliances. You've probably seen those final transformers as the gray oil-drum sized units on utility poles or the green boxes on front lawns. But despite all this effort to minimize power loss, something like 6% of the electric power generated in this country is lost in the delivery process.