Physicists know that electrons in a solid, such as a piece of rock, are always moving based on a combination of experimental evidence and theoretical understanding.
Experimental Evidence: Numerous experimental observations support the notion that electrons in solids are in constant motion. Here are a few examples:
a. Electrical Conductivity: Solids can conduct electricity due to the movement of electrons. When an electric field is applied to a solid material, the free electrons within the material can move and carry electric charge, resulting in the flow of current. This phenomenon, observed in conductors, indicates that electrons are mobile within the solid.
b. Thermal Conductivity: The thermal conductivity of solids is related to the movement of electrons. Electrons can transfer heat energy through collisions with other electrons and lattice vibrations (phonons). The efficient transfer of heat in conductive materials suggests that electrons are constantly participating in thermal motion.
c. Quantum Hall Effect: The Quantum Hall Effect is a phenomenon observed in two-dimensional electron systems subjected to a strong magnetic field. The Hall resistance in such systems is quantized, indicating the presence of discrete, individual charge carriers (electrons). The quantization of Hall resistance provides evidence for the mobility and movement of electrons within the solid.
Theoretical Understanding: The behavior of electrons in solids is described by quantum mechanics and solid-state physics. Theoretical models, such as the band theory of solids, provide an understanding of the electronic structure and dynamics in materials. These models consider the energy levels available to electrons in the solid and the statistical distribution of electrons according to these levels.
In the band theory of solids, the concept of energy bands and band gaps explains the behavior of electrons. In insulators, the electrons are tightly bound within filled valence bands, and there is a significant energy gap between the valence band and the higher, unoccupied conduction band. In conductors, the valence and conduction bands overlap, allowing electrons to move freely.
Furthermore, quantum mechanics describes electrons as wave-particle entities, with wave functions that describe their probabilistic behavior. The uncertainty principle suggests that electrons cannot have both well-defined positions and velocities simultaneously. Thus, electrons are inherently in motion, even at low temperatures, due to their wave-like nature.
Taken together, the experimental evidence, along with the theoretical understanding of electrons' behavior in solids, indicates that electrons are always moving, contributing to various macroscopic properties and phenomena observed in materials.