Which material among the following is an insulator?

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

Answer: (A)

Electrical conductivity differentiates insulators from conductors. An insulator has high resistivity and resists the flow of current, while a conductor offers many free charge carriers and conducts readily. Rubber fits the insulator role because its electrons are tightly bound and there are few mobile charge carriers, so applying a voltage does not produce a significant current. That's why rubber is used as insulation on cables. Copper and aluminum are metals with many free electrons, so they conduct electricity very well. Saltwater contains dissolved ions that move when a voltage is applied, making it a good conductor as well. Therefore the material that acts as an insulator in this list is rubber.