[sdiy] Alternating dead current voltage??

[RMC]

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.

 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.

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


 RMC, England