1

Equilibration of fluid levels and pressures

A

If we fill two tanks to different levels with oil or water, and if we connect them through a hose at the bottom (Figure 1, Movie 1), we observe that the level which is higher decreases, while the one that is lower increases. This happens as long as the levels are different. The process runs fast at the beginning and then slows down. This happens no matter how big—or how wide—the tanks are, or what shape they have.

Interpretation

Liquids are stored in tanks, and they can flow from tank to tank. For the liquid to flow, level differences are needed. We say that a level difference drives the flow of liquid. When the level difference is high (as at the beginning), the flow is high. Inflow and outflow of liquids lead to changes of the quantities stored in the tanks. A strong net flow will lead to a fast change of the quantity stored.

B

If oil is filled in one of two communicating tanks, and water in the other, levels will be different after equilibration has taken place (Photo 1). In another experiment, two balloons are connected by a hose having a valve (Photo 2). Both balloons are filled with air, one to a high pressure, the other to a somewhat lower one. When the valve is opened, air flows from the balloon having the higher air pressure into the other one.

Interpretation

Nature does not equilibrate levels or volumes but the pressure of fluids. This is seen when pressure gauges at the bottoms of the tanks in Figure 1 are used. The same result is obtained if different liquids are filled into the tanks as in the U-pipe of Photo 1, or if the pressure of air in communicating balloons is measured (Photo 2, diagram of pressures in Figure 2).

Consequently, we can interpret the pressure difference in the communicating containers as the driving force for flows.