12.1 Flow rate and its relation to velocity

Learning objectives

By the end of this section, you will be able to:

• Calculate flow rate.
• Define units of volume.
• Describe incompressible fluids.
• Explain the consequences of the equation of continuity.

The information presented in this section supports the following AP® learning objectives and science practices:

• 5.F.1.1 The student is able to make calculations of quantities related to flow of a fluid, using mass conservation principles (the continuity equation). (S.P. 6.4, 7.2)

Flow rate $Q$ is defined to be the volume of fluid passing by some location through an area during a period of time, as seen in [link] . In symbols, this can be written as

where $V$ is the volume and $t$ is the elapsed time.

The SI unit for flow rate is ${\text{m}}^3\text{/s}$ , but a number of other units for $Q$ are in common use. For example, the heart of a resting adult pumps blood at a rate of 5.00 liters per minute (L/min). Note that a liter    (L) is 1/1000 of a cubic meter or 1000 cubic centimeters ( ${\text{10}}^{-3}{\text{m}}^3$ or ${\text{10}}^3{\text{cm}}^3$ ). In this text we shall use whatever metric units are most convenient for a given situation.

Flow rate and velocity are related, but quite different, physical quantities. To make the distinction clear, think about the flow rate of a river. The greater the velocity of the water, the greater the flow rate of the river. But flow rate also depends on the size of the river. A rapid mountain stream carries far less water than the Amazon River in Brazil, for example. The precise relationship between flow rate $Q$ and velocity $\overline{v}$ is

where $A$ is the cross-sectional area and $\overline{v}$ is the average velocity. This equation seems logical enough. The relationship tells us that flow rate is directly proportional to both the magnitude of the average velocity (hereafter referred to as the speed) and the size of a river, pipe, or other conduit. The larger the conduit, the greater its cross-sectional area. [link] illustrates how this relationship is obtained. The shaded cylinder has a volume

which flows past the point $\text{P}$ in a time $t$ . Dividing both sides of this relationship by $t$ gives

We note that $Q=V/t$ and the average speed is $\overline{v}=d/t$ . Thus the equation becomes $Q=A\overline{v}$ .