Voltage vs. Current — and Why Current Doesn't Get "Used Up"

Two quantities that get treated as interchangeable and aren't. One is a difference between two points. The other is a flow around a loop — and it comes back out exactly as it went in.

Voltage is a difference in electric potential between two points — it doesn't exist "in" a wire on its own, only between two locations you've chosen to compare. Current is the actual movement of charge, measured througha point in a circuit. Mixing these up is how people end up believing current "drains away" as it passes through a light bulb — as if some of it simply vanishes. It doesn't. In a simple series loop, the current is the same at every point around it, from start to finish.