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Voltage :
Voltage is something is a type of "pressure" that drives electrical
charges through a circuit.
Bodies with
opposite
charges attract, they exert a force on each other pulling them
together. The magnitude of the force is proportional to the product
of the charge on each mass. This is just like gravity, where we use
the term "mass" to represent the quality of bodies that results in
the attractive force that pulls them together.
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Electrical force, like gravity, also depends inversely on the
distance squared between the two bodies; short separation means big
forces.
Thus it takes an opposing force to keep two charges of
opposite sign apart, just like it takes force to keep an apple from
falling to earth. It also takes
work and the expenditure of energy to pull positive and
negative charges apart, just like it takes work to raise a big mass
against gravity, or to stretch a spring. This stored or potential
energy can be recovered and put to work to do some useful task.
A
falling mass can raise a bucket of water; a retracting spring can
pull a door shut or run a clock. It requires some imagination to
devise ways one might hook on to charges of opposite sign to get
some useful work done, but it should be possible.
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Fig - 1 :
Opposite charges exert an attractive force on each other
The potential that separated opposite charges have for doing work if
they are released to fly together is called voltage, measured in
units of volts (V). 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." |
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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 , VAB
, is relative. If VAB 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, VAB and VAC to your common point A .
You can even determine the voltage between B and C without measuring
it: VBC = VAC - VAB . 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 VAB
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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