VOLTAGE

As explained in the previous section, to move the electron in a conductor in a conductor in a particular direction requires some work or energy transfer. This work is done by an external electromotive force (emf), typically represented by battery. This emf is also known as potential difference or voltage. Actually, whenever positive and negative charges are separated, energy is expanded.
Voltage is the energy per unit charge created by the separation. Thus, the voltage V₁₂ between two point 1 and 2 in an electric circuit is the energy or work needed to move a unit charge from 1 to 2. We express this ratio in differential form as :-
v=V₁₂=dw/dq................ (1.3)

where, 

w=the energy in joule

q=the charge in coulombs

v=V₁₂=the voltage in volts

From equation (1.3) it is evident that
 1 volt=1 joule/coulomb=1 newton-meter/coulomb

Thus, voltage or potential difference is the energy required to move a unit charge through an element.

Figure below shows the voltage across a lamp connected between point 1 and 2.

The plus (+) and minus (-) sign s are used to represent reference direction or voltage polarity. The voltage V₁₂ can be interpreted in two ways. 

1. Point 1 is at a potential of V₁₂ volts higher than point 2.
2. The potential at point 1 with respect to potential 2 is V₁₂.

Therefore, logically it follows that,

In figure (a), point 1 is +10V above point 2; in figure (b), point 2 is -10V above point 1. We can say that in figure (a) there is a 10V voltage drop from point 1 to 2 or equivalently a 10V voltage rise from point 2 to 1. In general, a voltage drop from 1 to 2 is equivalent to a voltage rise from 2 to 1.