This article is about electrical conductivity in general. Ohm’s law for dummies pdf piece of resistive material with electrical contacts on both ends. Every material has its own characteristic resistivity. For example, rubber has a far larger resistivity than copper.
If the pipes are the same size and shape, the pipe full of sand has higher resistance to flow. It also depends on the length and width of the pipe: short or wide pipes have lower resistance than narrow or long pipes. The resistance of a given material increases with length, but decreases with increasing cross-sectional area. The above definition was specific to resistors or conductors with a uniform cross-section, where current flows uniformly through them. The electrons instead “fill up” the band structure starting from the bottom. In contrast, the low energy states are rigidly filled with a fixed number of electrons at all times, and the high energy states are empty of electrons at all times. In metals there are many energy levels near the Fermi level, meaning that there are many electrons available to move.
This is what causes the high electronic conductivity of metals. An important part of band theory is that there may be forbidden bands in energy: energy intervals that contain no energy levels. In insulators and semiconductors, the number of electrons happens to be just the right amount to fill a certain integer number of low energy bands, exactly to the boundary. In this case, the Fermi level falls within a band gap. Since there are no available states near the Fermi level, and the electrons are not freely movable, the electronic conductivity is very low. This is also known as a positive ionic lattice.