Which of the following is an insulator?

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

Answer: (A)

An insulator is a material that resists the flow of electric current because it has very high resistivity and few free charge carriers to carry charge under an electric field. Rubber fits this description because its electrons are tightly bound and there are hardly any free electrons or ions available to move when a voltage is applied. That prevents current from passing through easily, making rubber a good electrical insulator used to protect people and devices.

In contrast, metals like copper and aluminum have lots of free electrons that move easily when a voltage is applied, so they conduct electricity. Saltwater contains dissolved ions that can move and carry charge, so it also conducts electricity well. If rubber gets wet, it can lose some of its insulating properties, but dry rubber remains a solid insulator.