Answer to Question #301693 in Electricity and Magnetism for Chemistry

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?

Answer 1

Answer

When the dielectric strength exceeded for the material the dielectric breakdown occurs.

The dielectric breakdown creates holes in the Mylar. The holes in the Mylar are distributed by air.

When dielectric breakdown takes place, dielectric becomes a conductor. Therefore, the Mylar becomes a conductor. The area of the holes in the Mylar film is a less fraction of the total area in the conductor. Now the presence of air makes small difference in the electric field between the plates.

The dielectric strength of air is less than the dielectric strength of Mylar.

When the electric field is strong, dielectric breakdown takes place. Therefore, when the dielectric strength of air exceeded the breakdown occurs at the air.