[sdiy] Alternating dead current voltage??

[Magnus Danielson]

From: "RMC" <RMC at richardcraven.plus.com>
Subject: Re: [sdiy] Alternating dead current voltage??
Date: Fri, 24 Dec 2004 13:59:42 -0000
Message-ID: <019301c4e9c0$d1406ef0$0201a8c0 at cruncher>

> Magnus, all
> 
> >> The thing is that we stopped thinking about AC as standing for 
> >> Alternating
> >> Current and just think of it as a qualifier stating that we have shifting
> >> polarities.
> >> So, when we say AC Voltage we intend to say voltage of shifting
> >> polarity when we incorrectly is saying alternating current voltage, which 
> >> is
> >> wrong as you correctly have identified. This is a misuse of the language
> >> really. It would be better to say Alternating Voltage and Direct Voltage
> >> (altought Direct isn't a good word). Even better would be Alternating and
> >> Non-Alternating Current/Voltage.
> >
>  It isn't a misuse of language but it is a misunderstanding of what that
>  language means.  When we talk about voltages, we are really referring to a
>  current flowing in a resistance or reactance due to a difference in
>  potential energy of charges i.e. a potential difference exists due to an
>  electromotive force.

Actually, no, you only cover part of the meaning we are refering to, the way
you express things. The AC/DC aspect is given to supply sources. Their
effective (RMS) voltage is roughly the same all the time, since for many cases
the source impedance is low compared to the potential load we are gonna toss
it. The current we pull will however vary. So, it is a voltage source capable
of supporting a range of current loads rather than the opposite. If we where
talking about a current source capable of supporting a range of voltages as a
load is added, then Alternating or Direct Current would be very appropriate.
However, since the primary mode of operandi when designing is voltage driven
these days, and currents becomes what they becomes, then it is a bit
misleading.

Volt, a Potential difference in electromotive force (really the potential
energy per unit charge), is the potential to do something. Current, the flow
of unit charges per unit time, is the amount of fullfillment of this potential.

>  So, an AC voltage is a voltage measured across something, as a result of an
>  Alternating Current (which forces charge backwards and forwards through a
>  resistance). A DC voltage is one where the driving current is unipolar, and
>  although the magnitude of it might change, the direction of current flow
>  (and hence the subsequent voltage measured) does NOT vary.

Sadly, I don't think this is very helpfull. While insightfull it doesn't really
help out in the field, and for linear passive curcuits (resistors, capacitors
and inductors) is is certainly of no help but rather confusing.

What is important is the shift of poles, which is really shifting the potential
of the poles so that they alternate between being lower and higher in potential
than the other. We could be arguing about weither the current or the voltage is
the reference, but they are complementary and not necessarilly in phase. What
is important is that we have either a voltage or current source which either
have a non-alternating or alternating characteristic in its voltage or current
sign, this is what we are trying to convey in at least 99,99999% of the time,
regardless if it is a current or voltage source.

> > Superconductors have current but no voltage and are very cool to fool 
> > around
> > with. I want a superconductor to operate in +70 C, that could be very 
> > usefull!
> 
>  Strictly speaking this is true - there is no "voltage" because there is 
> zero
>  resistance across which to develop that potential difference. However, 
> there
>  must be an EMF - electromotive force - established at some point in order
>  for current to begin to flow.

I agree. Superconductors in themselfs is just non-lossy conductors, but the
wish for a current to flow come outside of the superconductor system itself.
Infact, it is usually done though induction.

>  I feel it is important to understand what voltages, currents, resistances
>  are - otherwise misunderstanding of these basics becomes a huge source for
>  confusion.

Yes, but on the same time has the actual use of terms deviated from what
motivated them. Actually, some terms came about outside of the physics labs
and theory departments and where generated in the engineering departments as
people needed them. Missleading or not, we're stuck with them and trying to
forge a correct scientific explanation to them will only add to the confusion
rather than solve things. I beleive that the AC and DC are good examples of
this. The negatively charged electron is another of those little misshaps that
didn't go corrected. The Celsius scale was corrected thought, now accepted in
its shifted form as Kelvin scale for absolute reference.

Cheers,
Magnus