electricresistance

Electric resistance is a property of a material or component that opposes the flow of electric current. It causes a voltage drop and, in most cases, converts some electrical energy into heat. The unit of resistance is the ohm (Ω). The reciprocal of resistance is conductance.

In direct current circuits, resistance R relates voltage and current through Ohm's law: V = IR, or equivalently I = V/R. The power dissipated by a resistor is given by P = VI = I^2R = V^2/R. In alternating current circuits, resistance is part of a broader concept called impedance, which also accounts for reactive effects from inductance and capacitance; a pure resistor in AC analysis exhibits resistance with no phase difference between voltage and current.

Resistance depends on material, temperature, length, and cross-sectional area. For a uniform material, R = ρL/A, where ρ is resistivity measured in ohm-meters and depends on the material and its temperature. The conductivity is the reciprocal of resistivity. Temperature changes affect resistivity: ρ(T) ≈ ρ0[1 + α(T − T0)], where α is the temperature coefficient of resistivity.

Geometric factors influence resistance: longer conductors have higher resistance, while thicker conductors have lower resistance. Materials vary widely: metals typically have low resistivity, making them good conductors, whereas insulators have high resistivity. Semiconductors show more variable resistivity and are central to electronic components. Resistors used in circuits are designed to provide specific resistance values and stable performance within operating conditions.

Applications include current limiting, voltage division, sensing, and heating elements, where resistance determines how a component controls current and dissipates energy.