This page introduces resistance as the ratio of potential difference to current and relates it to material properties, dimensions, and temperature.
In symbols, $$R = \dfrac{V}{I}$$ and it is measured in ohms (\(\Omega\)), with \(1\,\Omega = 1\,\mathrm{V\,A^{-1}}\).
The resistance of a uniform wire depends on its length \(L\), cross‑sectional area \(A\), and the material’s resistivity \(\rho\). For a wire of length \(L\) and cross‑sectional area \(A\), $$R = \dfrac{\rho L}{A}$$
Doubling the length doubles the resistance, while doubling the area halves it. For circular wires, \(A = \pi r^2\), so doubling the diameter significantly decreases the resistance by a factor of 4.
For metallic conductors, resistivity and hence, resistance increases with temperature. As temperature rises, lattice vibrations increase and electron scattering becomes more frequent, producing a higher effective resistance (positive temperature coefficient).
An ohmic conductor obeys Ohm’s law at constant temperature: \(V \propto I\). Its I–V graph is a straight line through the origin and the resistance \(R = V/I\) (the slope) is constant.
The filament is a metal (typically tungsten) with a positive temperature coefficient of resistivity. As current flows, the filament heats up. Higher temperature increases lattice vibrations. Electrons scatter more, their mobility drops, and resistivity rises.
A diode allows current to flow primarily in one direction: it conducts after a forward threshold and blocks in reverse (apart from a tiny leakage). Applications include rectifiers in power supplies, polarity protection, signal detection, and light emission in LEDs (special diodes that emit light under forward bias).
A diode allows current to flow primarily in one direction: it conducts after a forward threshold and blocks in reverse (apart from a tiny leakage).