Basic concepts
What are fluids?
In physics, fluid is a term that refers to gases and liquids together. In most situations, gases and liquids behave similarly and we can apply the same physical toolset to describe them. But how would one define this unifying concept – what is a fluid?
- Something that can flow?
- Something that has a definite volume but has no defined shape?
Though these attempts at a definition a rather close to the mark, the rigorous physical definition is based on the concept of shear stress.
Fluid
A substance that cannot support shear stress.
Compressibility
Pressure applied to an object can change its physical dimensions (see volume stress). Fluids can differ in compressibility — whether external pressure can change their volume and thus their density.
Compressibility of fluids
Incompressible
Fluids whose density is constant regardless of the pressure exerted on them.
Compressible
Fluids whose density may be changed if the external pressure changes.
Liquids are generally incompressible. Whilst gases themselves are compressible, the flow of gases can be considered incompressible at low flow speeds (under \(50\,\mathrm{m}/\mathrm{s}\)).
Example
If you fill a syringe with water and stop the opening, you are unable to push the piston to any distance, whereas if there is air in the syringe, the piston can move further towards the opening.
Viscosity (internal friction)
Though fluids cannot support shear stress, they do resist shearing motion. The motion of layers within the fluid is hindered by internal friction, which is a result of molecular interactions. The measure of internal friction is called viscosity.
Viscous and non-viscous fluids
Viscous fluid
The internal friction within the fluid is significant.
Non-viscous fluid
The internal friction within the fluid is negligible.