An insulator is a material that resists the flow of electric current. Rubber, glass, plastic, and dry wood are common insulators. They have few free electrons, so charge cannot move through them easily. Insulators are essential to electrical safety. The plastic coating on a wire keeps current from flowing into your hand. The glass or ceramic insulators on power lines keep current from flowing into the pole. Without insulators, electricity would be impossible to control.

Insulators also resist the flow of heat. Materials like fiberglass, foam, and wool trap air, and air is a poor conductor of heat. That is why a down jacket keeps you warm and a thermos keeps your coffee hot. In electronics, insulators are used to separate conductive parts and prevent short circuits. The best insulators have high resistivity, meaning they resist the flow of charge. But no insulator is perfect. Apply enough voltage and the material breaks down, allowing current to flow. That is what happens when lightning strikes a tree or when a spark jumps across a gap.

Insulator facts

• Definition. Material that resists electric current.
• Examples. Rubber, glass, plastic, dry wood.
• Thermal. Also resist heat flow.
• Safety. Used to coat wires and line power poles.
• Breakdown. Enough voltage can overcome any insulator.

An insulator is not the same as a semiconductor. A semiconductor conducts under certain conditions. An insulator does not conduct under normal conditions.