Newbie that don't understand

[Jim]

Peter Hansen wrote:
> Just a dude wrote:

>>My son is an electronics
>>design engineer, and I still can't get my head around negative
>>voltages, how the hell do you get a -12 V supply... to him it's basic
>>but to me..............

> With a 12V supply, the power supply is trying to push electrons
> out the negative terminal and into the positive terminal.  If you
> have a -12V supply, it tries to push electrons out the positive
> terminal and into the negative terminal.

While this is way off topic and way pedantic, electrons
always leave and enter the same terminals (think electro-
chemistry, like a battery). Electrons leave the negative
terminal and flow towards the positive terminal (blame
this on whoever decided protons were positive and electrons
negative).  The electro-motive force (emf, which is what
voltage  measures) that causes this to happen is measured
as a difference in potential. The + terminal is 12v above
the - terminal.

If you connect the + terminal of a 12v battery to ground
(which is by definition a zero potential or 0v reference
point), the - terminal will be 12v below ground (the 12v
difference and polarity of difference are unchanged). IOW
12v volts below ground (0v) will be -12 volts. Some British
sports cars from the 50's and 60's have the positive
terminal connected to the chassis (ground for a car),
so they have a -12v (-6v?) electrical system (or "positive
ground"). There is no difference, however, between the
battery in an American car and these British cars - just
a difference in reference point. As Einstein said, it's
all relative..


Jim