In the previous topic we said "the cell provides a push that drives the current" and deliberately stopped — we promised to name and measure that push later. This is later. So as you read, keep one wire from the last topic in your hand: current is the flow; potential difference is the push behind the flow. Two different quantities, two different units, two different meters.
1. Present the physical scene
Picture a water tank on the roof of a house — the kind you fill in the morning before the supply cuts off. A pipe runs from the tank down to a tap in the kitchen. Open the tap and water rushes out. Strong flow.
Now picture the same tank, but only a few centimetres of water left at the bottom, barely above the pipe. Open the same tap. The water dribbles out, weak and slow.
Same tap, same pipe. The only thing that changed is how high the water sits — the height difference between the tank and the tap. A big height difference pushes the water hard. A small one barely pushes at all.
Stop scrolling. Try it in your head before reading on. In both cases the pipe and tap are identical. So what is it that makes the water flow strong in one case and weak in the other?
(Answer: the height difference between the water level and the tap. A tall column of water presses down hard — a big "push" — so water flows fast. A nearly-empty tank gives a tiny height difference, a tiny push, a weak flow. The push is not the water and not the flow-rate; it is the difference in height that drives the water. In an electric circuit, the cell creates a difference of exactly this kind — a difference that pushes charge — and that difference is what we are about to define.)
You can now name the scene we are about to explain: charge needs a "push" to flow, and that push is set by a difference the cell creates between two points — just like the height difference drives the water.