Dielectric strength

When the external electric field applied to a dielectric becomes very large, it can tear the bound electrons away from their parent atoms, converting the previously bound (immobile) charges into free charges; once this happens, the dielectric starts to conduct electricity, a phenomenon called dielectric breakdown. The maximum electric field a given dielectric material can withstand before this breakdown occurs is called its dielectric strength. Air, for example, has a dielectric strength of about 3x10^6 V/m; if the applied field exceeds this, the surrounding air itself breaks down and a visible spark is produced (the underlying cause of the spark seen when static electricity discharges, or the much larger spark of a lightning strike, discussed further in section 1.9.3). Table 1.1 lists the dielectric strengths of several common insulating materials -- mica (100x10^6 V/m), teflon (60x10^6 V/m), paper (16x10^6 V/m), air (3x10^6 V/m), and pyrex glass (14x10^6 V/m) -- showing why materials s …