Re: what is dielectric strength and dielectric stress??

Consider a parallel plate capacitor. A dielectric (nonconductor) is often put between the plates for two reasons -- one electric; the other mechanical. The electric reason is the molecules within the dielectric may increase the capacitance because the dielectric permittivity is higher than air's. The mechanical reason is to keep the plates apart. A charged capacitor's plates attract each other. The dielectric provides a reaction force keeping the plates apart. It thus feels a mechanical compressive stress. Dielectric strength, according to Electronic Engineers' Handbook, is " ... the maximum (electric) field strength that a dielectric can sustain without breakdown." Electric field strength is measured in volts per meter. What is breakdown? In this case, it's the condition in which the dielectric becomes a conductor, albeit unwilling. Electrons are ripped from the dielectric atoms and their motion is a conduction current. Dielectrics ordinarily do not allow this. Dielectric stress is the electrostatic force divided by the area. It is not a mechanical breakdown limit. Mechanisms for mechanical dielectric failure under voltage stress differ over the long and short term and depend on the dielectric. One long-term failure mechanism reported more than 20 years ago was the growth of metallic "whiskers" that penetrated the dielectric and shorted the plates. The whiskers were perfectly formed crystals that proved to have high mechanical strength. Very nice, but they caused the capacitor to fail. Bother the strength — the things conducted current too! Lawrence Skarin Professor, Electrical/Telecommunications Technology Monroe Community College Rochester, NY