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Electronics >> Basics of Electronics >> Voltage


What is Voltage?

Voltage 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.

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.

Opposite charges exert an attractive force on each other

                                   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).


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 , 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.

Like charges exert a repulsive force on each other
                           
Fig - 42: Like charges exert a repulsive force on each other.

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