Chapter 10: Resistance and Resistivity

The chapter begins by exploring Ohm's law, which states that the current flowing through a conductor is directly proportional to the applied potential difference, a relationship demonstrated through I-V characteristic graphs where ohmic components such as metallic conductors display linear relationships. However, many real-world components deviate from this ideal behavior. Non-ohmic components including filament lamps exhibit increased resistance at higher temperatures as the metal filament heats up and undergoes thermal vibration that impedes electron flow. Thermistors, particularly negative temperature coefficient devices, show dramatically reduced resistance when heated, making them valuable for temperature sensing applications. Diodes and light-emitting diodes conduct only when forward-biased and exceed a threshold voltage, while remaining nearly insulating in reverse bias. Light-dependent resistors further demonstrate how environmental factors influence conductivity, with resistance dropping significantly under increased illumination. The chapter then explains the physical origins of resistance in both metallic conductors and semiconductors. In metals, free electrons drift through the material but collide with vibrating positive ions, with collision frequency and energy loss increasing as temperature rises. Semiconductors function through a contrasting mechanism where electrons transition from bound states to free conduction as temperature or light energy increases. The chapter concludes by introducing resistivity, a material property that quantifies how strongly a substance opposes current flow. Resistivity depends on material composition and temperature but is independent of shape. The relationship between resistance and resistivity is expressed through the resistivity equation, where resistance is directly proportional to length and inversely proportional to cross-sectional area. The chapter also briefly discusses superconductivity, an extreme state at very low temperatures where certain materials achieve zero resistance and enable applications like magnetic resonance imaging and particle accelerators.