The water tank
Before the next quantity is named, here is the picture the whole section rests on — and you already know it from plumbing rather than from physics.
Raise the tank and the water runs faster
You know that as the height at which a water tank is fitted increases, the speed at which water flows along the pipe increases. The flow gets faster because the pressure difference between the tank and the place the water is drawn from is greater.
| In the water system | In the electric circuit |
|---|---|
| The tank, raised up high | The electric source — a cell or a supply |
| The pressure difference between the two ends of the pipe | The electric pressure difference the source sets up by pushing electrons from its negative terminal, through the conductor, to its positive terminal |
| The water flowing along the pipe | The current flowing round the circuit |
A flow needs a difference, not just a supply
Water does not run because there is water; it runs because one end is higher than the other. That is the point of the analogy, and the activity at the end of this skill is built to prove the same thing for a circuit.
It is only an analogy
Water is used up at the tap; charge is not used up in a circuit. The comparison is there to make difference feel natural, and it should not be pushed further than that.