As we have seen it a previous post a power supply has an electromotive force (e.m.f). The e.m.f. is the energy that the power supply will provide to 1 C of charge to make the 1 C of charge move around the circuit.
Now in a circuit there may be circuit components. Fig 1 below shows a circuit where there is a power supply and a bulb.
Now the 1 C is moving from the positive terminal and is moving around the circuit. While passing through the bulb it will move form the point A to the point B. Now the 1 C of charge has electrical energy. When it will pass through the bulb it will lose the electrical energy which will be transformed into another form.We say that the energy is dissipated.
Hence we can define the potential difference between point two points is the energy dissipated when 1 C of charge move from one of the point to the other.
Hence if the potential difference across the bulb is 3 V it means that when 1 C of charge flows from point A to point B 3 J of energy is dissipated in the bulb.
Hence if the potential difference across a circuit element is V and Q charge flows in the circuit element then the energy dissipated W can be calculated using the equation
W = VQ