VOLTAGE

Voltage or potential difference is the energy required to move a unit charge through an element. It is measured in volts (V). The instrument used to measure voltage between two points is known as voltmeter.

Another term used in conjunction with voltage is 'EMF'. But what is EMF?

EMF stands for electromotive force. It is defined as the potential difference that exists between the terminals of a battery when it is not connected to a load (i.e. the cell is on 'no load'). 

SYMBOLS USED

Consider a charge +Q in space. The electric field due to this charge is coming out of it as defined conventionally. Let a unit charge 'q' be made to move from point A to B. Then, the energy move this unit charge from point A to B is defined as the voltage V_ab.

In other words, Voltage or potential difference is defined as the amount of work that is needed to be done per unit charge, to move the charge from one point to another (i.e. A to B).

Mathematically,\[V=\frac{W}{q}\]

i.e. 1 Volt = 1 Joule/coulomb = 1 Nm/Coulomb

According to ohm's law, voltage between two points is given by the product of current through that path and the resistance offered by the path.

\[V=IR\]

Consider the figure given below:

It shows that 'a' is at a higher potential than point 'b'. This means that there is a drop of 9 volts from point a to b.

TYPES OF VOLTAGE - 

Like electric current, a constant voltage is called a DC voltage and is represented by V, whereas a sinusoidally time varying voltage is called an AC voltage and is represented by v.

The voltage produced by a battery is DC voltage and that produced by an electric generator is AC voltage.

HYDRAULIC ANALOGY - 

The word 'voltage' is analogous to 'pressure' and 'tension'. Consider two tanks filled with different heights of water in them and connected to each other. Then, the tank in which water is filled to a greater height will have more potential than the other tank with lower water height. Water will flow from tank A to tank B as the tap is opened.This is because of the potential difference between the two tanks.

The above mentioned example best illustrates a battery in which there is difference of potential between positive and negative terminals which causes the flow of charges from one terminal to the other until the charges get neutralised.
Similarly, other terms such as current and resistance can also be explained using water analogy method for a more clear and better undderstanding.