# Electrical reactance

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In electrical and electronic systems, reactance is the opposition of a circuit element to a change in current or voltage, due to that element's inductance or capacitance. The notion of reactance is similar to electrical resistance, but it differs in several respects.

An electric current is a flow of electric charge. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

In electromagnetism and electronics, inductance describes the tendency of an electrical conductor, such as coil, to oppose a change in the electric current through it. The change in current induces a reverse electromotive force (voltage). When an electric current flows through a conductor, it creates a magnetic field around that conductor. A changing current, in turn, creates a changing magnetic field, the surface integral of which is known as magnetic flux. From Faraday's law of induction, any change in magnetic flux through a circuit induces an electromotive force (voltage) across that circuit, a phenomenon known as electromagnetic induction. Inductance is specifically defined as the ratio between this induced voltage and the rate of change of the current in the circuit

## Contents

In phasor analysis, reactance is used to compute amplitude and phase changes of sinusoidal alternating current going through a circuit element. It is denoted by the symbol ${\displaystyle \scriptstyle {X}}$. An ideal resistor has zero reactance, whereas ideal inductors and capacitors have zero resistance that is, respond to current only by reactance. The magnitude of the reactance of an inductor rises in proportion to a rise in frequency, while the magnitude of the reactance of a capacitor decreases in proportion to a rise in frequency. As frequency goes up, inductive reactance also goes up and capacitive reactance goes down.

In physics and engineering, a phasor, is a complex number representing a sinusoidal function whose amplitude (A), angular frequency (ω), and initial phase (θ) are time-invariant. It is related to a more general concept called analytic representation, which decomposes a sinusoid into the product of a complex constant and a factor that encapsulates the frequency and time dependence. The complex constant, which encapsulates amplitude and phase dependence, is known as phasor, complex amplitude, and sinor or even complexor.

A sine wave or sinusoid is a mathematical curve that describes a smooth periodic oscillation. A sine wave is a continuous wave. It is named after the function sine, of which it is the graph. It occurs often in pure and applied mathematics, as well as physics, engineering, signal processing and many other fields. Its most basic form as a function of time (t) is:

Alternating current (AC) is an electric current which periodically reverses direction, in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.

## Capacitive reactance

A capacitor consists of two conductors separated by an insulator, also known as a dielectric.

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axes align to the field.

Capacitive reactance is an opposition to the change of voltage across an element. Capacitive reactance ${\displaystyle \scriptstyle {X_{C}}}$ is inversely proportional to the signal frequency ${\displaystyle \scriptstyle {f}}$ (or angular frequency ω) and the capacitance ${\displaystyle \scriptstyle {C}}$. [1]

Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.

In physics, angular frequencyω is a scalar measure of rotation rate. It refers to the angular displacement per unit time or the rate of change of the phase of a sinusoidal waveform, or as the rate of change of the argument of the sine function.

Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components.

There are two choices in the literature for defining reactance for a capacitor. One is to use a uniform notion of reactance as the imaginary part of impedance, in which case the reactance of a capacitor is a negative number: [1] [2] [3]

${\displaystyle X_{C}=-{\frac {1}{\omega C}}=-{\frac {1}{2\pi fC}}}$

Another choice is to define capacitive reactance as a positive number, [4] [5] [6]

${\displaystyle X_{C}={\frac {1}{\omega C}}={\frac {1}{2\pi fC}}}$

In this case however one needs to remember to add a negative sign for the impedance of a capacitor, i.e. ${\displaystyle Z_{c}=-jX_{c}}$.

At low frequencies a capacitor is an open circuit so no current flows in the dielectric.

A DC voltage applied across a capacitor causes positive charge to accumulate on one side and negative charge to accumulate on the other side; the electric field due to the accumulated charge is the source of the opposition to the current. When the potential associated with the charge exactly balances the applied voltage, the current goes to zero.

Driven by an AC supply (ideal AC current source), a capacitor will only accumulate a limited amount of charge before the potential difference changes polarity and the charge is returned to the source. The higher the frequency, the less charge will accumulate and the smaller the opposition to the current.

## Inductive reactance

Inductive reactance is a property exhibited by an inductor, and inductive reactance exists based on the fact that an electric current produces a magnetic field around it. In the context of an AC circuit (although this concept applies any time current is changing), this magnetic field is constantly changing as a result of current that oscillates back and forth. It is this change in magnetic field that induces another electric current to flow in the same wire (counter-EMF), in a direction such as to oppose the flow of the current originally responsible for producing the magnetic field (known as Lenz's Law). Hence, inductive reactance is an opposition to the change of current through an element.

For an ideal inductor in an AC circuit, the inhibitive effect on change in current flow results in a delay, or a phase shift, of the alternating current with respect to alternating voltage. Specifically, an ideal inductor (with no resistance) will cause the current to lag the voltage by a quarter cycle, or 90°.

In electric power systems, inductive reactance (and capacitive reactance, however inductive reactance is more common) can limit the power capacity of an AC transmission line, because power is not completely transferred when voltage and current are out-of-phase (detailed above). That is, current will flow for an out-of-phase system, however real power at certain times will not be transferred, because there will be points during which instantaneous current is positive while instantaneous voltage is negative, or vice versa, implying negative power transfer. Hence, real work is not performed when power transfer is "negative". However, current still flows even when a system is out-of-phase, which causes transmission lines to heat up due to current flow. Consequently, transmission lines can only heat up so much (or else they would physically sag too much, due to the heat expanding the metal transmission lines), so transmission line operators have a "ceiling" on the amount of current that can flow through a given line, and excessive inductive reactance can limit the power capacity of a line. Power providers utilize capacitors to shift the phase and minimize the losses, based on usage patterns.

Inductive reactance ${\displaystyle \scriptstyle {X_{L}}}$ is proportional to the sinusoidal signal frequency ${\displaystyle \scriptstyle {f}}$ and the inductance ${\displaystyle \scriptstyle {L}}$, which depends on the physical shape of the inductor.

${\displaystyle X_{L}=\omega L=2\pi fL}$

The average current flowing through an inductance ${\displaystyle \scriptstyle {L}}$ in series with a sinusoidal AC voltage source of RMS amplitude ${\displaystyle \scriptstyle {A}}$ and frequency ${\displaystyle \scriptstyle {f}}$ is equal to:

${\displaystyle I_{L}={A \over \omega L}={A \over 2\pi fL}.}$

Because a square wave has multiple amplitudes at sinusoidal harmonics, the average current flowing through an inductance ${\displaystyle \scriptstyle {L}}$ in series with a square wave AC voltage source of RMS amplitude ${\displaystyle \scriptstyle {A}}$ and frequency ${\displaystyle \scriptstyle {f}}$ is equal to:

${\displaystyle I_{L}={A\pi ^{2} \over 8\omega L}={A\pi \over 16fL}}$

making it appear as if the inductive reactance to a square wave was about 19% smaller ${\displaystyle X_{L}={16 \over \pi }fL}$ than the reactance to the AC sine wave:

Any conductor of finite dimensions has inductance; the inductance is made larger by the multiple turns in an electromagnetic coil. Faraday's law of electromagnetic induction gives the counter-emf ${\displaystyle \scriptstyle {\mathcal {E}}}$ (voltage opposing current) due to a rate-of-change of magnetic flux density ${\displaystyle \scriptstyle {B}}$ through a current loop.

${\displaystyle {\mathcal {E}}=-{{d\Phi _{B}} \over dt}}$

For an inductor consisting of a coil with ${\displaystyle \scriptstyle N}$ loops this gives.

${\displaystyle {\mathcal {E}}=-N{d\Phi _{B} \over dt}}$

The counter-emf is the source of the opposition to current flow. A constant direct current has a zero rate-of-change, and sees an inductor as a short-circuit (it is typically made from a material with a low resistivity). An alternating current has a time-averaged rate-of-change that is proportional to frequency, this causes the increase in inductive reactance with frequency.

## Impedance

Both reactance ${\displaystyle \scriptstyle {X}}$ and resistance ${\displaystyle \scriptstyle {R}}$ are components of impedance ${\displaystyle \scriptstyle {Z}}$.

${\displaystyle Z=R+jX}$

where:

• ${\displaystyle Z}$ is the impedance, measured in ohms;
• ${\displaystyle R}$ is the resistance, measured in ohms. It is the real part of the impedance: ${\displaystyle {R=\Re {(Z)}}}$
• ${\displaystyle X}$ is the reactance, measured in ohms. It is the imaginary part of the impedance: ${\displaystyle {X=\Im {(Z)}}}$
• ${\displaystyle j}$ is the square root of minus one, usually represented by ${\displaystyle i}$ in non-electrical formulas (${\displaystyle j}$ is used so as not to confuse the imaginary unit with current, commonly represented by ${\displaystyle i}$).

When both a capacitor and an inductor are placed in series in a circuit, their contributions to the total circuit impedance are opposite. Capacitive reactance ${\displaystyle \scriptstyle {X_{C}}}$ and inductive reactance ${\displaystyle \scriptstyle {X_{L}}}$ contribute to the total reactance ${\displaystyle \scriptstyle {X}}$ as follows.

${\displaystyle {X=X_{L}+X_{C}=\omega L-{\frac {1}{\omega C}}}}$

where:

• ${\displaystyle \scriptstyle {X_{L}}}$ is the inductive reactance, measured in ohms;
• ${\displaystyle \scriptstyle {X_{C}}}$ is the capacitive reactance, measured in ohms;
• ${\displaystyle \omega }$ is the angular frequency, ${\displaystyle 2\pi }$ times the frequency in Hz.

Hence: [3]

• if ${\displaystyle \scriptstyle X>0}$, the total reactance is said to be inductive;
• if ${\displaystyle \scriptstyle X=0}$, then the impedance is purely resistive;
• if ${\displaystyle \scriptstyle X<0}$, the total reactance is said to be capacitive.

Note however that if ${\displaystyle \scriptstyle {X_{L}}}$ and ${\displaystyle \scriptstyle {X_{C}}}$ are assumed both positive by definition, then the intermediary formula changes to a difference: [5]

${\displaystyle {X=X_{L}-X_{C}=\omega L-{\frac {1}{\omega C}}}}$

but the ultimate value is the same.

### Phase relationship

The phase of the voltage across a purely reactive device (a capacitor with an infinite resistance or an inductor with a resistance of zero) lags the current by ${\displaystyle \scriptstyle {\pi /2}}$ radians for a capacitive reactance and leads the current by ${\displaystyle \scriptstyle {\pi /2}}$ radians for an inductive reactance. Without knowledge of both the resistance and reactance the relationship between voltage and current cannot be determined.

The origin of the different signs for capacitive and inductive reactance is the phase factor ${\displaystyle e^{\pm j{\pi \over 2}}}$ in the impedance.

{\displaystyle {\begin{aligned}{\tilde {Z}}_{C}&={1 \over \omega C}e^{j(-{\pi \over 2})}=j\left({-{\frac {1}{\omega C}}}\right)=jX_{C}\\{\tilde {Z}}_{L}&=\omega Le^{j{\pi \over 2}}=j\omega L=jX_{L}\quad \end{aligned}}}

For a reactive component the sinusoidal voltage across the component is in quadrature (a ${\displaystyle \scriptstyle {\pi /2}}$ phase difference) with the sinusoidal current through the component. The component alternately absorbs energy from the circuit and then returns energy to the circuit, thus a pure reactance does not dissipate power.

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## References

1. Shamieh C. and McComb G., Electronics for Dummies, John Wiley & Sons, 2011.
2. Meade R., Foundations of Electronics, Cengage Learning, 2002.
3. Young, Hugh D.; Roger A. Freedman; A. Lewis Ford (2004) [1949]. Sears and Zemansky's University Physics (11 ed.). San Francisco: Addison Wesley. ISBN   0-8053-9179-7.
1. Irwin, D. (2002). Basic Engineering Circuit Analysis, page 274. New York: John Wiley & Sons, Inc.
2. Hayt, W.H., Kimmerly J.E. (2007). Engineering Circuit Analysis, 7th ed., McGraw-Hill, p. 388
3. Glisson, T.H. (2011). Introduction to Circuit Analysis and Design, Springer, p. 408
4. Horowitz P., Hill W. (2015). The Art of Electronics , 3rd ed., p. 42
5. Hughes E., Hiley J., Brown K., Smith I.McK., (2012). Hughes Electrical and Electronic Technology, 11th edition, Pearson, pp. 237-241
6. Robbins, A.H., Miller W. (2012). Circuit Analysis: Theory and Practice, 5th ed., Cengage Learning, pp. 554-558