# Is there a type of current other than AC and DC

1. Oct 2, 2016

well we have learned in school there are two types of current DC whichs direct and AC where the curve change dir alternately. so what if the the graph is not a sine curve or a cos curve. is there any other name for such types of current?

2. Oct 2, 2016

### haruspex

No direct current current is perfectly constant, and no alternating current is a perfect sine wave. A current which deviates significantly from the ideal in each case you might call merely non-reversing or reversing, respectively. (I just made those terms up.)

3. Oct 2, 2016

### Simon Bridge

There is just "current" ... since i(t) can be any function at all.
The current in an RC circuit (whether charging or discharging) is neither DC nor AC.
Specific names are usually for the shape of the function.... but we study sinusoids because we can construct pretty much any real current from a sum of sine waves, so if we know how sine waves work, we can deduce how most any wave will work.

4. Oct 2, 2016

### pixel

Isn't it DC, as it's in one direction, and exponentially decreasing with time?

5. Oct 2, 2016

### Simon Bridge

Well done ... how about white noise? It changes direction but randomly.
Can you come up with one?

6. Oct 2, 2016

### Staff: Mentor

In ordinary usage, AC is understood as being a single-frequency sinusoidal voltage and DC is understood as constant voltage. Thus, anything else is neither AC nor DC, although if it is reasonably close we will describe it as poor-quality AC or noisy DC.

7. Oct 2, 2016

### pixel

But here on Physics Forums we are not ordinary.

8. Oct 3, 2016

### Jehannum

Isn't this just one of those questions that does nothing but start arguments about arbitrary categorisations?

9. Oct 3, 2016

### haruspex

The question in post #1 is whether there is an existing standard terminology here. I think the responders so far are agreed there is not.

10. Oct 3, 2016

### sophiecentaur

Categorising things gets in the way of understanding. Long ago we (in Science) stopped using 'forward / backwards / stopped to describe the motion of an object. We use distance / speed and a sign. Zero speed or distance is all part of the set. We use Vectors to describe things. The maths becomes much more straightforward when you do it that way.
Moreover, pure 'DC' is a mathematical abstraction. There is no source of DC that remains constant over all time. A battery voltage changes as it discharges / charges, as the temperature changes and in the short term (milli and microseconds) as the rate of chemical reaction changes microscopically.
AC and DC are terms used by Electrical Engineers to describe basic characteristics of electrical supply methods.

11. Oct 3, 2016

### pixel

DC means the current is in one direction only. It does not necessarily imply a constant current. That is the definition of DC and it is not an abstraction; a constant DC current is an abstraction.

Last edited: Oct 3, 2016
12. Oct 3, 2016

### sophiecentaur

If we want to get picky about this, then the term 'Unidirectional' describes current that flows in one direction only. The DC Component of any wave form is the unvarying part.

13. Oct 3, 2016

### pixel

14. Oct 3, 2016

### sophiecentaur

I accept that your version of "DC component" is in frequent use (Engineers' usage would be the term I would use) but, if it is changing then it has a non zero frequency component. In a waveform such as the one you are showing, there is a clear distinction between the 'wanted' AC waveform and any offset that may occur. Also, of course, there is never a 'DC' value that goes from -∞>t> ∞ (mathematical abstraction) but the context of what 'your' DC means can be different from 'my'DC mean. If You are a 50/60HZ power engineer then your DC timescale could be tens of seconds or more. If I am an RF engineer then my DC timescale could be a microsecond (one screen's worth on a high frequency 'scope).
Look at the spectrum of your signals or my signals and there is only one DC term ( at 0Hz) but there will be a bandwidth around that value which we choose to include for our purposes.
My problem and reason for posting here was that it seemed to be implied that 'any' signal which excurses only one side of the zero line should be described as DC. That is far too simple a description. If a waveform happens to be Unidirectional, that could well be of interest in an experiment.

Last edited: Oct 3, 2016