Electric potential difference or voltage

You may have already heard that electric potential difference or voltage is an electric pressure which causes current to flow in a closed circuit. What the question is how electric pressure is created?
Before understanding electric potential difference it is important to know that what electric potential is ?

Electric Potential

Electric potential give the idea of how much electric energy a specific area of the electric field provide.
Therefore, at any point in a electric field, the electric potential is the amount of electric potential energy divided by the amount of charge at that point.

Electric Potential = Electric Potential Energy/Charge

Therefore the unit of electric potential is Joule per coulombs(J/C), which is also called volt (V).

The charges body in an electric field have an electric potential energy.

Suppose that a positive test charge is placed in a electric field, it is repelled away from positive plates (higher potential) and attracted by negative plates (lower potential). When we try to move positive charge toward positive plate, it is repel by the positive plate. Therefore to move positive test charge against this repulsive force some work have to done. This work done is stored in the charged body in the form of electric potential energy. Therefore charged body has the capacity to do work by moving other charges either by attraction or repulsion.
To move the test charge more closer to positive plates, more work have to done, i.e. more energy gets stored. Similarly,  to pull a negative charge away from positive terminal against electric field work have done. This work done is store in the charged body in the form of electric potential energy and can be dissipate in external electric circuit.

The electric potential energy of an charged body describes how much stored energy it has and how much work it can do.

To make it more simple take an example,
Suppose, electric potential at a point is 20 volt.
It means that if we move one coulomb of charge from any point to that point we have to do work of 20 joule. This work done in the form of electric potential energy. Hence charged will have electric potential energy of 20 Joule.
Similarly if we move 3 coulomb of charge from any point to that point, the charge will have electric potential energy of 60 Joule.
Potential energy at a point is 20 joule means that potential energy per unit charge is 20 volt which means each column of charge has an energy of 20 Joule.

Electric potential difference or voltage

The difference in the electric potential between two charged body is called electric potential difference.
Electric potential difference is also called voltage.

Suppose we have two points, one has higher electric potential then another. When these points are joined through a conductor, electron will start flowing from lower potential  to higher potential. Hence the conventional current flow will be in the opposite direction.

There is an interesting point that why do electrons move from lower potential to higher potential the difference.
As we know that positive part has higher electric potential and negative part has lower electric potential. Since electrons are negative charge particles, they are repelled away from negative sides and attracted by the positive sides.

Inside the source, efforts are made to bring negative charge particles (electrons) from higher potential to lower potential and then keep stay electrons at lower potential. This effort (work done) is stored in the charge particle in the form of electric potential energy which may further dissipate in external circuit in the form of light, heat, sound etc.
When lower potential and higher potential side are connected through a external wire the electron start move from lower potential to higher potential.
When the electron is pulled away from positive side or higher electric potential, it try to move back to high electric potential. This will results in a force, which create electric pressure in circuit.

Unit of electric potential difference
Electric potential difference is generally expressed in terms of ‘Volt
The potential difference between two points is said to be 1 volt if 1 joule of work is done in transferring one coulomb of charge from one point to another.
In simple when one coulomb of change is move from lower potential to higher potential and it done a work of one joule then the potential difference is said to be one volt.