How is Voltage Measured?
The greater the amount of charge and
the greater the physical separation, the greater the voltage or
stored energy. The greater the voltage, the greater the force that
is driving the charges together. Voltage is always measured between
two points, in this case, the positive and negative charges. If you
want to compare the voltage of several charged bodies, the relative
force driving the various charges, it makes sense to keep one point
constant for the measurements. Traditionally, that common point is
called
"ground."
So how do you tell if a particular bunch of charge is positive or
negative? You can't in isolation. Even with two charges, you can
only tell if they are the same (they repel) or opposite (they
attract).
The names are relative; someone has to define which one is
"positive." Similarly, the voltage between two points A and B , V_{AB}
, is relative. If V_{AB} is positive you know the two points
are oppositely charged, but you cannot tell if point A has positive
charge and point B negative, or visa versa.
However, if you make a
second measurement between A and another point C , you can at least
tell if B and C have the same charge by the relative sign of the two
voltages, V_{AB} and V_{AC} to your common point A .
You can even determine the voltage between B and C without measuring
it: V_{BC} = V_{AC}  V_{AB} . This is the
advantage of defining a common point, like A , as ground and making
all voltage measurements with respect to it.
If one further defines
the charge at point A to be negative charge, then a positive V_{AB}
means point B is positively charged, by definition. The names and
the signs are all relative, and sometimes confusing if one forgets
what the reference or ground point is.
Fig  42:
Like charges exert a repulsive
force on each other.
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