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Impedance is a measure of the opposition to electrical flow in a circuit or part of a circuit. It is denoted by the symbol Z and measured in ohms. In electrical engineering, impedance is the result of the combined effect of resistance and reactance in a circuit. The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element. It is important to note that impedance is only observed for AC signals, not for DC signals, where impedance and resistance are the same.
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What You'll Learn
Impedance is the opposition to alternating current
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element, as determined by their relative amplitudes and phases. In a two-terminal circuit element, the impedance is the ratio of the complex representation of the sinusoidal voltage between its terminals to the complex representation of the current flowing through it. It depends on the frequency of the sinusoidal voltage.
The impedance of a circuit is influenced by its resistive (R) and reactive (X) components. The resistive component arises from collisions of the current-carrying charged particles with the internal structure of the conductor. The reactive component, on the other hand, is an additional opposition to the movement of electric charge due to changing magnetic and electric fields in circuits with alternating current.
Impedance can be calculated using the rules for combining impedances in series and parallel. For components connected in series, the total impedance is the sum of the individual component impedances. Impedance can be measured using an LCR meter, which measures a component's inductance (L), capacitance (C), and resistance (R). These values can then be used to calculate the impedance at any frequency.
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Impedance includes resistance and reactance
Impedance is a measure of the total opposition that a circuit or a part of a circuit presents to electric current. It is measured in ohms and is represented by the symbol Z. Impedance includes both resistance and reactance.
Resistance (R) is a force that tends to resist the flow of electrical current. It is created by a resistor to achieve some effect. The more resistive a resistor is, the more it restricts the flow of electricity through it. Resistance is usually measured in ohms and is defined as the voltage across an element divided by the current (R = V/I).
Reactance (X) is an additional opposition to the movement of electric charge that arises from the changing magnetic and electric fields in circuits carrying alternating current. It is an unfortunate by-product of certain electrical components. There are two basic types of reactance: capacitive reactance and inductive reactance. Capacitive reactance is created by capacitors, while inductive reactance is created by inductors. Reactance is also expressed in ohms and it behaves in much the same way as resistance, in that it tends to restrict the flow of current through a circuit.
The impedance of a circuit is the square root of the sum of the squares of the resistance and reactance. In other words, impedance can be represented as the hypotenuse of a right triangle, with resistance and reactance as the shorter sides.
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. It is useful for performing AC analysis of electrical networks, as it allows relating sinusoidal voltages and currents by a simple linear law.
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Impedance is measured in ohms
Impedance is a measure of the opposition to electrical flow in a circuit. It is represented by the symbol Z and measured in ohms. In simpler terms, impedance is the resistance to the flow of electrical current.
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element. This is determined by the relative amplitudes and phases of the voltage and current.
In a direct current (DC) system, impedance and resistance are the same, defined as the voltage across an element divided by the current (R = V/I). In other words, impedance reduces to resistance in circuits carrying steady direct current. Resistance arises from collisions of the current-carrying charged particles with the internal structure of the conductor.
In an alternating current (AC) system, the reactance enters the equation due to the frequency-dependent contributions of capacitance and inductance. The impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance, whereas resistance forms the real part. In AC circuits, impedance possesses both magnitude and phase, unlike resistance, which has only magnitude.
Impedance can be measured or displayed directly in ohms, or other values related to impedance may be displayed. For example, in a radio antenna, the standing wave ratio or reflection coefficient may be more useful than the impedance alone. The measurement of impedance requires the measurement of the magnitude of voltage and current, and the phase difference between them.
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Impedance is calculated using phasor voltage and phasor current
Impedance is a measure of the opposition to electrical flow in a circuit or a part of a circuit. It is calculated as the ratio of the phasor voltage to the phasor current at a pair of terminals in a circuit. Phasors are used by electrical engineers to simplify computations involving sinusoids, such as in AC circuits, where they can often reduce a differential equation problem to an algebraic one.
Phasor diagrams present a graphical representation, plotted on a coordinate system, of the phase relationship between the voltages and currents within passive components or a whole circuit. Phasors are defined relative to a reference phasor, which always points to the right along the x-axis. Sinusoidal waveforms of the same frequency can have a phase difference between themselves, which represents the angular difference of the two sinusoidal waveforms. The terms “lead” and “lag”, as well as “in-phase” and “out-of-phase”, are commonly used to indicate the relationship of one sinusoidal waveform to another.
The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element, as determined by the relative amplitudes and phases of the voltage and current. This is identical to the definition from Ohm's law, recognising that the factors of resistance and reactance contribute to the opposition of current flow in a circuit. In a DC circuit, the opposition to current flow is simply called resistance, whereas in an AC circuit, the presence of reactance leads to the concept of impedance.
The phasor technique is used to calculate voltages and currents in AC circuits. The main idea of the phasor technique is to reduce the operations of calculus on sinusoidal quantities to algebraic operations on their phasors. This idea can be extended to a broader class of voltages and currents. The real part of this frequency characterizes the exponential decay (or growth) of the peak value of the voltage, while the imaginary part is the angular frequency.
The use of an impulse response, in combination with the fast Fourier transform (FFT), can be used to rapidly measure the electrical impedance of various electrical devices. The LCR meter measures a component's inductance (L), capacitance (C), and resistance (R); from these values, the impedance at any frequency can be calculated.
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Impedance mismatches can cause issues in electrical systems
Impedance is a measure of the opposition to electrical flow in a circuit. It is the ratio of the complex representation of the sinusoidal voltage between a circuit's terminals to the complex representation of the current flowing through it. In simpler terms, it is the measure of the total opposition that a circuit or a part of a circuit presents to electric current.
In multiple port networks, the two-terminal definition of impedance is inadequate, and impedance mismatches can occur due to discontinuities in the physical path of transmission, improperly terminated lines, and sudden step discontinuities in impedance lines. This can lead to a reduction in the quality of the signal and impact the overall system functionality.
There are techniques to improve impedance mismatches, such as inserting a matched attenuator in front of a mismatched load impedance or adjusting the characteristic impedance of the feed line. Electrical engineers may also use equivalent impedances in calculations instead of dealing with the electromagnetic properties of an antenna.
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Frequently asked questions
Impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. It is a measure of the total opposition that a circuit or a part of a circuit presents to electric current.
The SI unit of electrical impedance is the ohm (Ω).
Impedance is calculated as the ratio of the phasor voltage across the element to the phasor current through the element. It can also be calculated using an LCR meter, which measures a component's inductance (L), capacitance (C), and resistance (R).
