Which of the following is an example of an insulator?

• A. Copper
• B. Rubber
• C. Water
• D. Aluminum

Answer: (B)

Electricity flows more easily through materials that have freely moving charged particles; those are conductors. Insulators resist the flow because their electrons are tightly bound and there are very few free charge carriers, giving them very high resistivity. Rubber fits this description well: its molecular structure holds electrons tightly, so even when a voltage is applied it doesn’t allow significant current to pass. That’s why rubber is used to coat wires and components to prevent unintended current flow and protect you from shocks.

Copper and aluminum are metals with many free electrons, so they conduct electricity readily. Water’s ability to conduct depends on impurities—ordinary water contains ions and can conduct, whereas very pure water conducts very poorly. Among the options, rubber is the clear insulator.