Permittivity is a property displayed by a substance where it has an innate ability to resist an induced electric charge within its structure. This external electric field is generally referred to as electric flux density, and materials that resist the formation of the field are known as dielectrics or, more commonly, insulators. The measurement of this property is done in units of the farad per meter as a measurement of electrical capacitance, and electric flux density is measured in coulombs per square meter, with a coulomb being defined as a value of electric charge that can be transferred in one second where a constant current of one ampere exists. Electric permittivity values are therefore expressed as a ratio of the imposed electric flux density versus the actual strength of the electric field that is generated in the material.
Measures of this property also vary depending on the medium and are often expressed as vacuum or relative permittivity. Vacuum permittivity is a pure measurement of the value in free or empty space, and is often referred to as the electric constant, or absolute or constant permittivity, and has an established value of 8.854187 x 10-12 farads per meter. Relative permittivity, on the other hand, is the representation of electric flux density in a medium as it directly compares to what that value would be in a vacuum, where absolute permittivity is unrestricted.
While values for permittivity can often be fairly straightforward to calculate, there are other factors in nature that also influence them and can result in complex permittivity. The generation of this property only occurs when an electric field is applied, so the result can often have varying phase differences in a medium based on the material itself and the angular frequency of the electric field. Other factors like temperature can also affect results, and the complex permittivity of water is one example where temperature and dissolved solutes such as with salt and other minerals in sea water vs. fresh water can have a significant impact on permittivity values. Changes in the permittivity of seawater are important for understanding the effects that the medium has on reflecting microwave radiation in global communications and for research in atmospheric science.
In modern electronics, permittivity factors are important in the creation of insulating materials. This becomes increasingly important as integrated circuit components become smaller and smaller, down to a molecular-atomic scale. Current leakage across transistors and other components in multi-layered circuit designs can degrade the circuit or keep it from functioning entirely. In contrast to this need for effective dielectrics, materials with strong permittivity values are useful in capacitors, computer memory, and other applications where electrical energy must be efficiently transferred and stored.