# What is a Voltage?

Well, we should really say potential difference... it's more descriptive.

To start with, it's not really a voltage at all!  That's a common, and acceptable, way of saying it, but it's kind of like saying poundage instead of money, or metreage instead of distance.  But it's so common, that even Physics examiners accept it!  So you can say that, but the correct term, the most correct term, is potential difference.

This is more useful in fact because the quantity is always measured as a difference between two points.  It is defined as the difference in energy per unit charge between two points.  And like so many things in Physics is best defined using an equation.

energy transferred (J) = charge (C) × potential difference (V)

So some components supply energy to charge and some transfer the energy from the charge.  Cells, power packs and photovoltaic cells are examples of components which supply energy to the charge in the circuit and lamps, resistors and motors are examples of components which transfer the energy.

One model of electricity imagines the charge as balls rolling around a track, cells raise the balls up and lamps allow the balls to roll down.  This is where the very useful idea of electrical slope comes from.  And in this model the potential difference is represented by the difference in heights before and after the component.

This is a law in Physics, we call it Kirchhoff's second law, or Kirchhoff's voltage law, and it states that the sum of the rises and falls in electrical potential will equal zero in a closed series circuit.