Consider a charge q is placed at a point. Let B and C be two points where point B is closer to q than C. If a charge q is brought from infinity to C or from infinity to B work done respectively be W_{C} and W_{B}.

The potential at points B and C respectively:

V_{B} = W_{B}/q

V_{C} = W_{C}/q

The potential difference is the difference in potentials V_{B} and V_{C}:

V_{B} - V_{C} = (W_{B} - W_{C})/q

W_{B} - W_{C} is the work done in carrying charge from point C to B.

Thus, **potential difference between two points** B and C is equal to the amount of work done in moving a unit charge from point C to point B.

**V = W/q**

The potential difference (pd) between two points of a conductor is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of charge from one point to another. Potential difference is a scalar quantity. It is measured using an instrument **voltmeter**. Voltmeter is always connected in parallel across which you have to measure the potential difference.

**Example 1:** How many electrons make one coulomb?

Let n electrons make 1 C.

Charge on 1 electron is 1.6 × 10^{-19} C.

Charge, q = +n|e|

n = q/e

= 1/(1.6 × 10^{-19})

= 6.25 x 10^{18} electrons

**Example 2:** Calculate the work done in moving a charge of 3 C across two points having a potential difference of 24 V.

Given, q = 3 C, V = 24 V, W = ?

W = qV

= 3 C x 24 V

W = 72 J