Chapter 25: Current, Resistance, and Electromotive Force

Electric current is defined as the rate of charge flow, measured in amperes, with current density relating this flow to the cross-sectional area through which charges move. The microscopic picture reveals that charge carriers, typically electrons in metals, acquire a small average drift velocity when subjected to an electric field, despite their otherwise random thermal motion. Material properties play a crucial role through resistivity and conductivity, with metals exhibiting low resistivity that increases with temperature, while superconductors display zero resistivity below critical temperatures. Ohm's law establishes the linear relationship between voltage and current through resistance, though many real devices exhibit non-ohmic behavior. The chapter introduces electromotive force as the energy per unit charge supplied by batteries and generators, distinguishing between ideal voltage sources and real sources that possess internal resistance, which affects terminal voltage and current flow in complete circuits. Energy and power considerations reveal how electrical energy is converted to heat in resistive elements, with power dissipation following specific mathematical relationships for different circuit components. The classical theory of metallic conduction explains resistance at the atomic level through electron collisions with ions, relating material resistivity to fundamental quantities like electron density and collision frequency. Circuit analysis techniques emerge naturally from these principles, providing tools to understand both simple and complex electrical networks, while phenomena like short circuits demonstrate the practical importance of resistance in controlling current flow and preventing component damage.