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# Difference Between Pressure and Flow

Pressure vs Flow

Pressure and flow are two terms often used when dealing with fluid; that is liquids or gasses. These two properties are characteristics of the state of fluid. Both fluid pressure and flow are point properties.

The pressure of a fluid is defined as the force acting per unit area within the fluid. Even though it refers to a unit area, it can be referred to the value of a point which is varying from point to point. It implies that the static fluid pressure is a point property. In SI units, pressure is measured by Pascal (Pa) or Newtons per square meter (Nm-2) and, in the imperial system, it is measured by pounds per square inch. Specifically, when measuring atmospheric pressure or gas pressure, mercury millimeters or mercury centimeters are also used. Pressure is not a vector quantity.

The pressure present within fluid can originate from two different factors. Static pressure is the pressure within a fluid when it is at rest, and the dynamic pressure is the pressure due to movement of the fluid. When considering the static pressure of gases and liquids, their sources are different. In liquids, the static pressure is caused by the weight of the fluid above the considered point and it varies with depth. In gasses, it is the rate of collision of gas molecules within the container. If the container is small, the gas pressure can be considered same at every point. If the gas has a larger volume, the weight also affects the static pressure (ex: atmospheric pressure).

On the other hand, the dynamic pressure of a fluid has its origin in the movement of the fluid, and it is closely related to the kinetic energy of the fluid (as reflected in the Bernoulli equation). In that context the static pressure is the potential energy of the fluid in a unit volume, and the dynamic pressure is the kinetic energy per unit volume.

Pressure and flow are related to each other, as pressure difference being the cause of the flow.

When a pressure difference between two points exists within a fluid and is not balanced out by the internal forces acting on the body, the fluid start to move from the higher pressure point to the low pressure point to minimize the pressure difference. This continuous movement of the fluid is known as flow.

Technically, flow refers to the amount of fluid passing through a given surface. This amount of flow can be measured using two parameters; i.e. the rate of volume flowing and the rate of mass flowing. The volume flow rate is defined as the volume of the fluid passing through a given surface in a unit time, and it is measured by cubic meters per second. The mass flow rate is defined as the mass passing through a given surface in unit time and measured by kilograms per second. Mostly, the term “flow” refers to the volumetric flow rate.

What is the difference between Pressure and Flow?

• Pressure is the force acting per unit area; it is a scalar point property of fluids.

• Flow is the rate at which fluid passes through a gives surface and flow is caused by the difference in pressure within a fluid.

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