Alternating Current

So far, we have looked at voltages and currents that are constant over time. The alternating voltage provided by a mains socket is sinusoidal and therefore not constant over time.

DC voltage of a battery (3.7 V) and sinusoidal AC voltage of the mains socket (±325 V, 20 ms) — Batt = battery · Steckdose = socket outlet

To distinguish “DC quantities” that are constant over time from “AC quantities”, we use upper-case letters for DC quantities and lower-case letters for AC quantities. AC quantities change over time; they are not constant. That is why we write an alternating voltage as u(t).

Why should we deal with AC quantities? Here are some fields of application from practice:

In medical technology, alternating fields for ultrasound, MRI etc. are generated from alternating current.
Smartphones communicate via WLAN or Bluetooth. Both radio standards use alternating currents to make digital information (“0” or “1”) distinguishable. The carrier frequency of 2.4 GHz describes a parameter of AC quantities: the frequency.
In power engineering, the conversion between electrical and kinetic energy in motors and generators is mostly done with alternating current. Every mains socket supplies an alternating voltage.