Which of the following materials is not an example of an insulator?

Copper is identified as a conductor, not an insulator, due to its high ability to allow the flow of electrical current. Metals, such as copper, possess free electrons that facilitate electrical conductivity, making them ideal for use in wiring and electrical applications.

In contrast, the other materials mentioned—rubber, wood, and silk—are characterized as insulators because they do not permit electrons to flow freely. Insulators are used to prevent unwanted current flow, thus protecting users from electric shock and ensuring the safety of electrical devices. Rubber is often used to encase wires, wood can serve as a base material for tools or devices, and silk, being a natural fiber, has very low conductivity. Understanding the differences between conductors and insulators is crucial in fields related to electrical safety and efficacy.