# AC, DC, what else?

Aside from alternating current and direct current we already know, do you still other type of current flow? Also, is it for that type of current flow to exist? or there are only 2 types of current flow in this universe (ac,dc)

• There is AC, DC, and European. If you're traveling from North America to Europe, you'll find they have differnt plugs and voltages and stuff. – Olin Lathrop Sep 13 '12 at 11:13
• @Olin That's a pretty naive and ignorant view you've taken. If we're sticking with just mains sockets, Australia has different sockets to both the US and Europe, for example. As does England, as well as the US using 110v and most other places using a more sensible 230/240v. – Bojangles Sep 13 '12 at 13:24
• @Olin - I would have expected that you would have said "AC, DC and French" :-) – stevenvh Sep 13 '12 at 13:28
• @Jam: Geesh, lighten up. I thought everyone would understand it was a joke. – Olin Lathrop Sep 14 '12 at 11:09
• @Olin Nothing in your tone hints that it was a joke. Don't tell me to lighten up if you don't have any helium handy. – Bojangles Sep 14 '12 at 11:20

In the DC/AC class there's nothing else. The criterion is frequency, and that can be zero (DC) or non-zero (AC).

Current is displacement of electrical charge, and that can be in different forms: electrons carry a negative charge, but cations for example a positive charge, so cations going the same direction as electrons will have a current with reverse polarity. That's another way to look at different kinds of current, but has nothing to do with the frequency.

So it's just AC and DC.

• so, any other ways of looking at it , aside from frequency and charge carriers? – WantIt Sep 13 '12 at 9:25
• @vvavepacket - sure. Speed, current density are the first ones which come to mind. – stevenvh Sep 13 '12 at 9:29

The way I get it, AC and DC are the names of two very specific current flows. They are named, because they are used by far the most often.

Image from wikipedia.

Forier transform is a mathematical method to represent any "good" function as a sum of a constant, sines and cosines. That is, any function, e.g. current, can be expressed as a combination of DC and (in)finitely many AC-s with different frequencies and ever smaller amplitudes. This could point out that there is only DC and AC.

However, this transform is not unique. Any set of "independent" (orthogonal) functions can be used as a base to decompose the original signal. Then there is "no" AC and DC.

Why do we use the Fourier decomposition so often? One reason is that sinusoids occur often in physical nature and thus are intuitive.

• But Fourier tells us that (almost) any real-world periodic signal, like your "pulsating", is just the combination of sinusoidal signals, like your "alternating", plus a d.c. component. – Joe Hass Sep 13 '12 at 11:17
• @Joe Hass, consider adding this to the answer if you approve the general idea of the answer. Which is "current can follow any function of time". For if we use fourier, there is no DC, only zero-frequency AC. What's more, fourier chooses sin and cos as a base. Other bases are possible. So there is no AC (sinusoidal current)? – Vorac Sep 13 '12 at 11:28
• @Vorac - the Fourier series starts with a term $a_0$/2, which is the average level of the signal, i.e. the DC component. – stevenvh Sep 13 '12 at 13:26
• @ Joe Hass, @stevenvh, tahnk you for this discussion. I have appended the answer - feel free to correct or edit it. /off lol - a nice picture always bring a ton of upvotes :D – Vorac Sep 13 '12 at 14:02
• "Why do we use Fourier decomposition so often?" --- Because for linear(ized) systems it allows us to solve complicated differential equations using only "simple" linear algebra. – The Photon Sep 13 '12 at 16:32