Question

A capacitor made of aluminum foil strips separated by Mylar film was subjected to excessive voltage, and the resulting Dielectric breakdown melted holes in the Mylar. After this, the Capacitance was found to be about the same as before, but the Breakdown voltage was much less. Why?

Short answer

The conductance of the air is much greater than the conductance of the Mylar film, then the current flow through the holes with a lower voltage than the previous situation.

Step-by-step solution

About Mylar Film

A form of polyester resin used to make heat resistant plastic films and sheets. It is made by copolymerizing ethylene glycol and terephthalic acid.

Determine why the breakdown voltage is less

As we know the dielectric breakdown generates holes in the dielectric. The holes are placed where Mylar film is filled with the air. As we know the area of holes is very tiny compared with the total area of the plate. Since the dielectric strength of air is much less than the dielectric strength of Mylar film. So, the capacitance will be the same as the previous case (before dielectric breakdown). As we know the conductance of air is much greater than the conductance of the mylar film, then current flows through the holes with a lower voltage than the previous situation ( before adding the air).So, the breakdown voltage will be much less than the previous situation.

Capacitance is proportional to the dielectric constant and is a geometric property of a configuration.

Therefore The conductance of the air is much greater than the conductance of the Mylar film, then the current flows through the holes with a lower voltage than the previous situation (before adding the air), So the breakdown voltage is much less than the previous situation.