Cylindrical Tank as a Constant Voltage Source

One of the main uses of a water storage tank is to maintain a water supply at constant hydrodynamic voltage.
Figure (1) is a schematic diagram of a typical town water supply. Water is pumped from the reservoir up into the water tank where a constant height h and therefore constant voltage ρgh is maintained. The houses in the town all draw constant voltage water from this tank.
Let us see what would happen if the water tank was too small. As soon as several houses started using water, the level h in the tank would drop and the pump at the reservoir would have to come on. The pump would raise the level back to h and shut off. Then the level would drop again and the pump would come on again. The result would be that the hydrodynamic voltage or water pressure supplied to the town would vary and customers might complain.
On the other hand if the town water tank has a large cross-sectional area and therefore large capacitance C, a few houses drawing water would have very little effect on the level h and therefore voltage ρg h of the water. The town would have a constant voltage water supply and the water company could pump water from the reservoir at night when electricity rates were low. We will see that one of the important uses of electrical capacitors in electric circuits is to maintain constant or nearly constant electric voltages. There is an accurate analogy to the way the town water tank maintains constant voltage water. If we use too small a capacitor, the electrical voltage will also fluctuate when current is drawn.

Figure 1: Town water supply. By maintaining a constant height h of water in the storage tank, all the water supplied to the town has a constant hydrodynamic voltage V_hρ gh .