What The Tech: What is the differ­ence between Direct current (DC) and alter­nating current (AC)

An elec­tric current is gener­ated when charge carriers perform a directed move­ment. Elec­tric currents flow in two different ways: either as direct current or as alter­nating current. If the flow of charge does not change direc­tion, this is direct current. In this case, there is also no change in polarity or current level. Direct current is used in mobile elec­trical appli­ances, for example. These usually have batteries or accu­mu­la­tors that supply a constant direct current. The active elec­tronics inte­grated in elec­trical appli­ances usually also require direct currents. This is why many stationary elec­trical appli­ances have recti­fiers that convert the alter­nating current of the Euro­pean power grid into pure direct current.

But for mass power supply in the indus­trial age, our power require­ments are often too high for batteries and accu­mu­la­tors. Ulti­mately, alter­nating current has prevailed and this is what comes out of power outlets in every house­hold today. An advan­tage of alter­nating current is that it can be produced in large quan­ti­ties. For example in hydro­elec­tric and wind power plants. Here, the rota­tion of perma­nent magnets gener­ates a changing magnetic field, and there­fore a change in polarity and alter­nating current.

The applied voltage can be increased and decreased flex­ibly and with low losses using a trans­former. The alter­nating current can then be easily distrib­uted to house­holds via distri­b­u­tion boxes. So-called ‘voltage losses’ are signif­i­cantly lower with alter­nating current than with direct current.