Science Puzzle
Why Do Power Lines Carry Such High Voltage?
The electricity in your home arrives at 230 volts. But the big pylons that carry it across the country run at up to 400,000 volts, and huge transformers are built at both ends of the journey just to step the voltage up and then down again.
Why go to all that trouble? Why not send the electricity at 230 volts the whole way?
The Answer
Because a high voltage lets the same power travel with a much smaller current, and a smaller current wastes far less energy heating the wires.
The power carried by a cable is the voltage multiplied by the current. So to deliver a given amount of power you can use a low voltage with a big current, or a high voltage with a small current. The wires themselves have a little resistance, and pushing current through them heats them up. The heat wasted grows with the square of the current: double the current and you lose four times as much energy.
Stepping 230 volts up to 400,000 volts makes the voltage about 1,700 times bigger, so the current for the same power is about 1,700 times smaller. The heat lost in the wires falls by 1,700 squared, roughly three million times. Sent at household voltage, most of the energy would warm the countryside long before it reached a town. At grid voltage, only a few per cent is lost.
Transformers, which make the stepping up and down possible, only work with alternating current, current that switches direction many times a second. That's a big part of why AC won the battle with direct current to power the world's electricity grids in the 1880s and 1890s.
The principle: Power loss in cables. Energy wasted as heat grows with the square of the current, so electricity is sent over long distances at very high voltage and low current, then transformed back down for use.