Difference Between Permeability and Porosity

渗透性vs Porosity

渗透性and porosity are two concepts discussed in many fields, in physics. These concepts also play a major role in some industries. Permeability is an important concept in fields such as electromagnetism, fluid mechanics, and earth science. Porosity is important in fields such as material science, geology, earth science, soil science etc. Porosity is also important in industries such as pharmaceutics, ceramics, and constructions. It is vital to have a proper understanding in permeability and porosity in order to excel in such fields. In this article, we are going to discuss what permeability and porosity are, their definitions, applications of permeability and porosity, the similarities between them, and finally the difference between permeability and porosity.

What is Permeability?

The term ‘permeability’ takes different meanings in different fields, but in general, permeability can be defined as the quality of a matter or membrane that decides the ability of that matter or membrane to allow liquids or gases to pass through. Vacuum permeability (or permeability in free space) and permeability in electromagnetism are two concepts that are widely used in physics. Before studying the vacuum permeability, it is important to get a good understanding about Ampere’s force law.

Think of a two thin, straight, stationary, parallel wires located with a distance r apart in free space. When a current I is carried in each wire, a force will be exerted on each other. Ampere’s law states that the force per unit length is given by F=µ₀I²/2πr, where force is denoted by F and vacuum permeability is denoted by µ₀. When the distance between the wires is 1 m, and 1 Ampere current flows in each wire, the force between the two wires is 2×10⁻⁷nm^-1. Hence, µ₀is equal to 4π ×10^-7NA^-2. In electromagnetism, permeability can be described as the measure of the ability of a material, to support the formation of magnetic field within itself. In electromagnetism, permeability is given by the equation B=µH, where permeability denoted by µ, magnetic flux density denoted by B, and magnetic field strength denoted by H. In earth science, permeability can be defined as the measure of the ability of a porous material, to allow fluids to pass through it. Here, SI unit of permeability is m².

What is Porosity?

孔隙率is a measure of the void or empty spaces in a material. This is also called void fraction in a material. The value of porosity falls between 0-1 or as a percentage between 0-100 %. Porosity of a material is given by the equation ø = V_V/V_T，在其中用Ø表示的孔隙率，v表示的空隙空间的体积_Vand total or bulk volume of material denoted by V_T. Materials such as granite have a low porosity compared with materials such as clay and peat. Several methods can be used to measure porosity. Those are direct methods, optical methods, Computed tomography method, water evaporation method, gas expansion method etc.

渗透性和孔隙率有什么区别？ •渗透性在不同领域（例如电磁，地球科学等）具有不同的含义，但孔隙率却没有。孔隙率是材料中空间空间的度量。 • Permeability has different SI units according to the fields it is applied. As an example, when it is applied in electromagnetism, its SI unit is NA-2, but in earth science, it is m2. Porosity has no such SI units; it has only a numerical value, which falls between 0-1. • Permeability is applied in many different fields such as electromagnetism, Amperes’ law and earth science, but porosity is applied in fields such as earth science, soil and mineral science etc.