Visualizing and measuring alternating current

Reading a sine wave on an oscilloscope

The oscilloscope displays voltage as a function of time and lets you directly see the sinusoidal shape of the alternating signal. You can read the amplitude Umax (height of the curve relative to zero) and the period T (duration of one complete pattern) from it, using the scale indicated on the device (volts/division and seconds/division).

Power in alternating current

For a pure resistor (such as a heater or an incandescent bulb), the average electrical power consumed is expressed using the RMS values:

P = Ueff * Ieff

Example: a heater plugged into the mains (Ueff = 230 V) drawing Ieff = 4 A dissipates an average power P = 230 * 4 = 920 W.

For more complex loads (motors, transformers), a power factor cos(phi) comes into play: P = Ueff * Ieff * cos(phi), with cos(phi) between 0 and 1.

Why electricity transport uses alternating current

Thanks to transformers, the voltage is stepped up (thus reducing the current for the same power) before transport on high-voltage lines, which greatly reduces losses due to the Joule effect (losses = R * I^2).

Common pitfall

Never calculate a power by directly multiplying the maximum values Umax and Imax: this would give a value twice as large as the real average power. Always use the RMS values Ueff and Ieff.