Fermi levels

The Fermi level is like a "top energy level" that electrons in a material tend to occupy at absolute zero temperature. It indicates the highest energy electrons have in a material when there are no thermal disturbances. In conductors, it's close to the energy levels allowing electrons to move freely, enabling electrical conduction. In insulators and semiconductors, it helps predict how electrons will behave—whether they'll stay put or move around when energy (like heat or voltage) is added. Essentially, the Fermi level provides a snapshot of a material's electron energy landscape.