Re: volts and amps

[Doug Craigen <dcc@ESCAPE.CA>]

John Denker wrote:
> At 08:19 AM 7/14/00 -0400, Wilson J. Gonzalez-Espada wrote:
>
> > I always thought that an electric appliance was designed for 4.5 V
> > (for example) AND a particular current.
>
> That statement is narrowly true, but may be misleading depending on context.

Yes, I've had problems for years with statements like "its high current
that can kill you not high voltage".  Such things appear to be true at
one level, but at another level they lead you down a wrong path in your
thinking including such things as not realizing where the voltage drop
is actually occuring in a given situation.  Going with this example
quoted above, in the mid 80's I was in a lab where we handled a few
voltage sources that went up to 100 kV.  Some of these were desktop
units for which you could leave wires exposed to the air and move your
hand in the general vicinity, the other was the size of a coffee table
and if you were foolish enough to come anywhere near an exposed wire you
would likely be struck by a bolt that could kill you.  The simple
explanation: the former were low current and the latter was high
current.  But in reality, in all cases there is a space charge
developing around the exposed wires and for the former the voltage of
the exposed wire dropped whenever significant current started to flow.

If a device is "designed for 4.5 V", to me that says its internal
resistance is much higher than that of the intended voltage source and
that voltage will normally dictate the current as a dependent parameter,
not as an independent parameter.

\_/~\_/~\_/~\_/~\_/~\_/~\_/~\_/~\_/~\_/~\_/

Doug Craigen
          http://www.dctech.com/physics/