Electricity: Units

A useful way to picture the difference between the quantities in this section is to imagine charge as water flowing through a pipe. Current is the rate the water flows past a point, measured in litres per second; charge itself is the total amount of water that has gone past, measured in litres. Potential difference plays the role of the pressure pushing the water along, and resistance is how narrow or obstructed the pipe is. None of these analogies replace the formal definitions, but they help explain why the equations connecting the units look the way they do: rate multiplied by time gives a total amount, in exactly the same way that current multiplied by time gives charge.