Weerstand in een AC-circuit

The alternating current source is a device that supplies a sinusoidally varying voltage or current with time.

A function can describe the sinusoidal voltage or current, where V or I are the voltage or current, respectively, at time t, V₀  or I₀ are the peak voltage or current, respectively, and ω is the angular frequency.

The sinusoidally varying voltages and currents are represented by rotating vector diagrams.

These vectors rotating counterclockwise at a constant angular frequency are called phasors; the diagrams depicting them are known as phasor diagrams.

Consider a resistor connected across an alternating current voltage source. Recalling Kirchhoff's loop rule, the instantaneous voltage across and the current flowing through the resistor can be determined.

Since the maxima and minima of both curves are reached simultaneously, the curves are said to be in phase.

Therefore, the relationship between current and voltage can be represented using phasor diagrams.

The projections on the vertical axis represent instantaneous voltage and current. Since they are in phase and have the same frequency, their phasors point in the same direction and rotate together.