Impedance measures how much a circuit opposes electricity flow. Resistance causes energy loss and reduces current flow. Capacitance and inductance contribute to impedance for AC. Capacitive reactance decreases with increasing frequency, while inductive reactance increases. Capacitors store and release electric charge, while inductors induce opposing currents. When combined, their effects depend on wiring and frequency. Impedance is measured in ohms and can be expressed as Z=V/I.
In electrical engineering, impedance is a measure of how much a circuit opposes the flow of electricity. All materials have some degree of electrical resistance, which causes 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 of which the circuit is made. For an alternating current (AC), however, two additional factors can contribute to impedance: capacitance and inductance. Together these are known as reactance, which is a measure of opposition to a change in current that depends on its frequency and circuit components.
Alternating current continuously changes direction, and does so at a given frequency, measured in Hertz (Hz), or cycles per second. Typically, electricity is supplied at 50 or 60Hz, but this can be modified for specific applications. Frequency can be viewed as a wave on an oscilloscope in terms of current or voltage, with the peak-to-peak distance representing one complete cycle. The degree of reactance in a circuit depends on the frequency of the AC supply. More specifically, the capacitive reactance decreases with increasing frequency, while the inductive reactance increases.
capacitive reactance
A capacitor is a device that can store an electric 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 in the insulator and effectively stores energy in an electric field. As the charge increases, the current decreases. After some time, the capacitor will be unable 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 it takes for the capacitor to “fill up”. Since the current is at its maximum at the beginning of a cycle, a high frequency AC supply will be virtually unaffected by a capacitor. Conversely, if the frequency is low, this will allow some charge to build up in the capacitor, causing the current to drop before the next cycle. Capacitors are used in many popular devices and gadgets, so capacitive reactance is usually a major factor in impedance.
inductive reactance
Inductance is the tendency of a changing current flowing through a wire to induce an opposing current in a nearby conductor. This happens because a changing electric current produces a changing magnetic field, which in turn causes electrons to flow into any conductive material within its range. When a wire is wound into a coil, it forms an inductor and will induce an opposing 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 wired. If a capacitor and inductor are connected in series, the current initially increases with frequency, reaching a maximum at a certain point, known as the resonant frequency, and then decreases thereafter. If they are connected in parallel, the current decreases with increasing frequency until it reaches a point where none flows. Beyond this point, the flow rises again.
Measure and unit
Like resistance, reactance and impedance are measured in ohms. In equations, impedance is usually represented by the symbol Z and reactance by X. Capacitive and inductive reactance are represented by XC and XL, respectively. Similar to Ohm’s law for resistance, overall impedance can be expressed as Z=V/I, where Z is expressed in ohms; V is the voltage, expressed in volts; and I is current, expressed in amperes.
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