viðnám

Viðnám is an Icelandic term meaning resistance or opposition. In everyday language it refers to resisting something or opposing motion or change. In technical usage, especially in physics and electrical engineering, it denotes the property of a circuit to oppose the flow of electric current, known as impedance.

Electrical impedance is a complex quantity Z that measures opposition to alternating current. It can be expressed as Z = R + iX, where R is resistance (the real part) and X is reactance (the imaginary part). Reactance arises from inductors and capacitors: X_L = ωL and X_C = -1/(ωC). The sign of X indicates whether inductive or capacitive effects dominate. The magnitude is |Z| = sqrt(R^2 + X^2) and the phase angle is φ = arctan(X/R), describing the lag or lead of current relative to voltage. In direct current, reactance vanishes for inductors and capacitors, so the impedance is real and equals the network’s resistance.

Ohm's law generalizes to V = IZ for impedance, where V and I are phasors. The real power is P = I^2R, the reactive power is Q = I^2X, and the apparent power is S = VI. The power factor is cos φ. Components with impedance include resistors, inductors and capacitors; networks combine impedances in series (Z_total = Z1 + Z2) or in parallel (1/Z_total = 1/Z1 + 1/Z2).

Outside electronics, viðnám can describe any resistance or opposition, such as social, political or physical resistance to change or external pressure.