what is the differences between electic pottential difference V & potential energy U?

Let us begin with the definition of those terms.

Electric potential: Electric potential at a point P is equal to the change in electric potential energy per unit test charge when it is moved from a reference point to the point P. It can also be defined as the work done per unit test charge from the reference point to the point P (provided the kinetic energy of the test charge is not changed).

V = kQ / r

Unit is Nm/C Electric potential is used mostly in the study of electrostatics.

 

Potential difference: Potential difference between two points is defined as the amount of work done in moving a unit positive charge from one point to another. Potential difference between two points is also equal to the product of the electric field and the distance between the two points.

V = E.d

It is measured in Voltage or simply volt (V).

Potential difference is mostly used in the study of current electricity.

 

Both electric potential and potential difference can be expressed in V and Nm/C.