A multimeter can be used to measure impedance.

In electrical engineering, impedance is a measure of the extent to which a circuit opposes the flow of electricity. All materials have some degree of electrical resistance, which causes some of the energy to be lost as heat and reduces the flow of current. In the case of direct current (DC), the impedance is equal to the resistance and depends exclusively on the materials with which the circuit is made. For an alternating current (AC), however, two additional factors can contribute to impedance: capacitance and inductance. Together they are known as reactance, which is a measure of opposition to a change in current that depends on its frequency and circuit components.

A power inverter, which can be used to convert DC to AC.

Alternating current constantly changes direction and does so at a certain frequency, which is measured in Hertz (Hz) or cycles per second. Electricity is typically supplied at 50 or 60 Hz, but this can be changed for specific applications. Frequency can be displayed as a wave on an oscilloscope in terms of current or voltage, with the crest-to-crest distance representing a complete cycle. The degree of reactance in a circuit depends on the frequency of the AC supply. More specifically, capacitive reactance decreases with increasing frequency, while inductive reactance increases.

capacitive reactance

A capacitor is a device that can store an electrical charge and then release it. It usually consists of a non-conductive, or insulating material, sandwiched between two metal plates. As part of a circuit, it allows a charge to build up on the insulator and effectively store energy in an electric field. As the load increases, the current is reduced. After a while, the capacitor will no longer be able to absorb any more charge and the current will drop to zero, at which point it will discharge, producing a flow of electrons in the opposite direction.

If, however, the AC frequency is high, the current will change direction in less time than the capacitor takes to “fill”. Since the current is at its maximum at the beginning of a cycle, a high frequency AC source will be virtually unaffected by a capacitor. In contrast, if the frequency is low, this will allow some charge to build up on the capacitor, causing a reduction in current before the next cycle. Capacitors are used in many popular devices and devices and therefore capacitive reactance is often an important factor in impedance.

Inductive reactance

Inductance is the tendency of a variable current flowing through a wire to induce an opposite current in a nearby conductor. This happens because a changing electric current produces a changing magnetic field, which in turn causes electrons to flow in any conductive material within its range. When a wire is wound around a coil, it forms an inductor and induces an opposite flow of electrons, or electromotive force (EMF) itself. The voltage of the induced EMF increases with the rate of change of the supply voltage, so increasing the AC frequency will increase the inductive reactance. Like capacitors, inductors are commonly used components.

Capacitors and inductors in combination

When both devices are present in a circuit, the effects depend not only on the AC frequency, but also on how they are connected. If a capacitor and an inductor are connected in series, the current initially rises with frequency, reaching a maximum at a certain point, known as the resonant frequency, and then falls. If they are connected in parallel, the current drops with increasing frequency until a point is reached where none flows. Beyond that point, the flow rises again.

measurement and units

Like resistance, reactance and impedance are measured in ohms. In the equations, impedance is normally represented by the symbol Z and reactance by X. Capacitive and inductive reactance are represented by XC and XL , respectively. As with Ohm’s law for resistance, the overall impedance can be expressed as Z = V / I, where Z is given in ohms; V is the voltage, expressed in volts; and I is current, given in amperes.