Answer
See below.
Work Step by Step
In an electric field, the atoms of an insulator are electrically distorted (as shown in Figure 22.11) as the electrons are pulled one way and the nucleus is pushed the other way. When the field strength increases past a critical level (the key term is "dielectric breakdown"), the atom suffers ionization, where the electrons and nucleus are physically separated from one another.
The ions and electrons are then free to move, and to conduct an electric current.
