What is a fluid?

We are constantly in contact with some substance that can be defined as fluid. Every day, for example, we need to drink a certain amount of water and we live “immersed” in a sea of ​​atmospheric air.

You fluids are defined as substances that have the ability to flow or flow, as they cannot resist a force which is parallel to its surface (shear stress), and always assume the shape of the container where they are confined. Some substances, such as honey and tar, take some time to take the shape of the container, but a fluid will always assume the same shape as the container in which it is inserted.

Liquids and gases are considered fluid because they do not have their atoms organized in a symmetrical and rigid way, forming a crystalline network, as happens with crystalline solids.

There are two quantities that characterize fluids: Especific mass and pressure.

• Especific mass

  The specific mass (ρ) is the result of the ratio between the mass of a fluid and the volume occupied by it:

  ρ = m
  V

  At the International System of Units, the unit of measure for specific mass is Kg/m³.

  As they are more compressible, the specific mass of gases varies a lot with pressure, which does not occur with liquids (see table).

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• Pressure

Pressure is the result of the action of the liquid on the walls of a container and can be defined from the ratio between the force applied by the fluid and the area of ​​application of that force:

P = F
THE

In the International System of Units, the unit of pressure is N/m², also called paschal (Pa). Other units used for this magnitude are the atm, which corresponds to the pressure exerted by the atmospheric air at sea level, and the Torr, name given in honor of Evangelista Torricelli because of the invention of the barometer. 1 Torr corresponds to the pressure generated by a 760 mm column of mercury.

1 atm = 1.01 x 10⁵ Pa = 760 Torr

Stevin's law determines the pressure exerted by a fluid at rest:

P = p_atm + ρ.g.h

P_atm = Atmospheric pressure;

ρ = specific mass of the liquid;

g = gravity;

h = Height of the liquid column considered.

By Joab Silas
Graduated in Physics

Would you like to reference this text in a school or academic work? Look:

JUNIOR, Joab Silas da Silva. "What is a fluid?"; Brazil School. Available in: https://brasilescola.uol.com.br/o-que-e/fisica/o-que-e-um-fluido.htm. Accessed on June 27, 2021.