Insulators constitute a class of substances, sometimes called dielectrics, that resist the flow of times electrical current. As such, they are useful in isolating electrical conductors one from another lating another. Insulators help prevent the loss of current from electrical circuits by electrically isolating circuits from grounded conductors. Insulators also help prevent electrocution—in which case one of the potential conductors is a living being—and they are an essential component of capacitors, which are electrical charge-storage devices.
Examples of insulators
Ceramics, such as glass and porcelain, are examples of rigid insulators; porcelain pots are used to suspend conductors from electricity-transmission towers, for example. Flexible insulators include polymers such as plasticized polyvinyl chloride (PVC), polytetrafluoroethene (PTFE), and polypropene (PP); these materials are used as flexible coatings for electrical cables.
Some polymers—the phenol-formaldehyde thermoset bakelite, for example—are tough materials that are useful for making rigid casings for electrical equipment. Hydrocarbons and certain inorganic compounds—notably sulfur hexafluoride (SF6)—are liquid insulators. They are useful for cooling and immersing electrical equipment in an insulating medium. Sulfur hexafluoride is used to quench electrical arcs between the contacts of one type of heavy-duty circuit breaker, for example. Other insulators include air, paper, mica (a mineral), and most metal oxides.
Resistivity and resistance
Resistivity is a measure of the intrinsic ability of a material to impede the flow of electrical current. As such, an ideal insulator—if one existed—would have an infinitely large resistivity.
Resistance, measured in ohms (Ω), is defined as the theoretical voltage required for a current of one amp to flow through a sample. In the case of insulators, however, the current is often much less than one amp, and the resistance is calculated by dividing voltage by current.
The resistivity, ρ (the Greek letter rho), of a material is derived from resistance measurements by compensating for the dimensions of samples of that material. If the distance between two electrodes separated by a resistive material doubles, for example, the resistance to the flow of current between those electrodes will also double. Resistivity is therefore measured as the resistance per unit length of sample. If the cross section of the sample doubles, the amount of current that flows at a given voltage doubles, and the resistance falls to half its original value. Resistivity is therefore calculated by multiplying resistance by the cross-sectional area of the sample.
Combining the two factors that account for sample dimensions, the resistivity of a material is calculated according to the following expression:
In this expression, ρ is resistivity (in Ωm), R is resistance (Ω), A is the cross section (m2) of the sample between two measuring electrodes, and l is the length (m) of sample between electrodes.
