Thumbnail: Electrons flowing in a metal according to Drude model with electrons (shown here in blue) constantly bouncing among heavier, stationary crystal ions (shown in red). (CC-BY-SA 3.0, Rafaelgar...Thumbnail: Electrons flowing in a metal according to Drude model with electrons (shown here in blue) constantly bouncing among heavier, stationary crystal ions (shown in red). (CC-BY-SA 3.0, Rafaelgarcia).