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So we have a DC level that, for example, varies between 0VDC and 10VDC in sinusoidal fashion. Is that classified as alternating current even though it does not go negative?

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  • \$\begingroup\$ good question..... \$\endgroup\$ – Trevor_G Jul 24 '17 at 16:37
  • \$\begingroup\$ It is DC-coupled AC. \$\endgroup\$ – Eugene Sh. Jul 24 '17 at 16:39
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    \$\begingroup\$ @Trevor Thanks - it bugged me when I first started EE more than 40 years ago! I know what it will do, but its classification always eluded me, not that it really matters. \$\endgroup\$ – Dirk Bruere Jul 24 '17 at 16:41
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    \$\begingroup\$ As the answers show, you look at nothing at all "absolutely" but only "relatively." This even goes to magnetic fields. To Sue, who is observing a moving charge, there is a resulting magnetic field. To Bob, who is traveling at the same frame of reference as the moving charge, there is no magnetic field at all. \$\endgroup\$ – jonk Jul 24 '17 at 16:51
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    \$\begingroup\$ it does go negative if you hang out in the middle... \$\endgroup\$ – dandavis Jul 24 '17 at 17:59
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It's all a matter of perspective and relativity. What you have is really a 10V Pk-Pk AC level superimposed on a 5V DC level. So the answer is yes. But I agree the term alternating is muddy in this case.

However, when we remember that voltage is a relative term it makes more sense.

If we think of AC like the classic point on a wheel spinning round the sinusoidal AC part is easy to imagine. But if we extend that and mount that wheel on top of a moving trolley we get your scenario. If you are standing on the trolley you see the dot going backwards, if you stand on the ground, you don't.

The term "Modulation-Current" might be more appropriate..

Which brings up a another question...

Why do we say alternating current not alternating volts.. which is the units we are usually quoting? e.g 110V AC

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  • \$\begingroup\$ Ah. But from the point of view of some other point sitting at 5 V DC, it's just 10 V AC and no DC at all. Or from the point of view of yet another point that is in-phase 10 V AC but centered on the ground reference point, it's just 5 V DC and that's all. So I like your "perspective and relativity" preface. \$\endgroup\$ – jonk Jul 24 '17 at 16:47
  • \$\begingroup\$ @jonk yup..answer was a work in progress \$\endgroup\$ – Trevor_G Jul 24 '17 at 16:47
  • \$\begingroup\$ The current may actually be changing direction even when the voltage is not changing polarity. \$\endgroup\$ – Chris Stratton Jul 24 '17 at 17:20
  • \$\begingroup\$ Good point @ChrisStratton.. and just to make it even more confusing... current can be AC even on a DC source. \$\endgroup\$ – Trevor_G Jul 24 '17 at 17:27
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AC with DC offset - So you have a 10V AC signal with a +5V DC offset

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    \$\begingroup\$ Or a 5V DC with a 10-volt AC component riding on it. Take your pick. Generally the choice depends on which component is more useful to you, or which has the larger value. \$\endgroup\$ – WhatRoughBeast Jul 24 '17 at 16:44
  • \$\begingroup\$ Or some weird signal riding on a square wave.. :) \$\endgroup\$ – Eugene Sh. Jul 24 '17 at 16:46
  • \$\begingroup\$ Fair point - I tend to think AC first but that's clearly just my brain and the fact we commonly encounter it that-way-around in the industry I work in (Thin Films and Vacuum Deposition)... But yes if it was 2V Pk-Pk on 5V DC I would think about it in the reverse \$\endgroup\$ – Rendeverance Jul 24 '17 at 16:47
  • \$\begingroup\$ Thinking about it further its interesting that we would attribute AC to a sine but we would likely, at least in my experience, not mention 'AC' unless the polarity was indeed switching, when its triangular or square wave - (i.e. 'and we applied a DC signal with a triangular waveform', or we applied 'pulsed DC', etc...) \$\endgroup\$ – Rendeverance Jul 24 '17 at 17:23

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