Conductors and Insulators

Why is a metal wire usually coated with plastic or rubber? The metal wire is an electrical conductor. The rubber and plastic are electrical insulators. An electrical conductor is a material through which charge can flow easily. A material through which charge cannot flow easily is called an electrical insulator. The coating around a wire helps to control the current and keep it where it is needed.

A metal is made up of ions in a lattice. The ions are not free to move. But each ion has one or more electrons that are not tightly bound to it. These free electrons can conduct charge. Most materials do not easily conduct charge because they don't have free electrons. Metals such as copper and silver are good electrical conductors. Wood, plastic, rubber, and air are good electrical insulators.

Figure 8 Using a thick straw to drink a milkshake is easier than using a thin straw. Similarly, electrons flow more easily through a thick wire than they flow through a thin wire, assuming the wires are made of the same material.

Applying Concepts Why should the wire in light bulb filaments be very thin?

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Resistance

As electrons move through a conducting wire, they collide with electrons and ions. These collisions convert some kinetic energy into thermal energy. Because less energy is available to move electrons through the wire, the current is reduced. Resistance is opposition to the flow of charges in a material. The SI unit of resistance is the ohm.

When you drink a milkshake as shown in Figure 8, it is easier if you use a thicker straw. In the same way, resistance is lowered if you make a wire thicker because more electrons can flow through a thicker wire. A material's thickness, length, and temperature affect its resistance. Resistance is greater in a longer wire because the charges travel farther. As temperature increases, a metal's resistance increases because electrons collide more often.

If resistance increases as temperature increases, what happens as you cool a conductor? Could you reduce the resistance to zero? This is the idea behind superconductors. A superconductor is a material that has almost zero resistance when it is cooled to low temperatures. The best superconductor found thus far must be cooled to about 138 K.