Materials that are good conductors of electricity have many “free” electrons. Here we mean free in the sense that they are not bound strongly to a given atom or molecule. Metals are often good conductors of electricity. Whether or not a material has freely conducting electrons is related to the detailed electronic structure of the material, and a thorough description would be beyond the scope of this book. Basically, the more electrons that are free to move around in a material, the more easily it can carry current, and the higher its conductivity.