ACkredsløb

ACkredsløb refers to electrical circuits in which voltages and currents vary with time, typically following sinusoidal waveforms at a chosen frequency. In steady state, an alternating-current (AC) analysis often uses phasors and impedance to simplify calculations. An AC source produces V(t) = Vpeak sin(ωt) or a rms value Vrms, with angular frequency ω = 2πf.

The key concept is impedance Z, which generalizes Ohm’s law to AC: V = I Z. For linear passive components, Z = R + jX, where XL = ωL is the inductive reactance and XC = 1/(ωC) is the capacitive reactance (with XC negative in the overall Z). In a series circuit, impedances add; in parallel, their reciprocals add.

Power in AC circuits is described by real power P, reactive power Q, and apparent power S. P = I^2R = VI cosφ, Q = VI sinφ, and S = VI, where φ is the phase angle between voltage and current. The power factor PF = cosφ indicates how much power does useful work.

Common configurations include RC, RL and RLC circuits. Resonance in a series RLC circuit occurs at ω0 = 1/√(LC), yielding minimal impedance and maximal current (depending on damping). These circuits are used in filtering, tuning, and impedance matching.

AC circuits differ from DC circuits in that voltages and currents change with frequency, reactive elements temporarily store energy, and average energy in capacitors and inductors is zero over a cycle. Analysts use phasor methods, and for transients Laplace transforms can be employed.