What is Reactance and Impedance
Impedance meaning – It is the measure of overall opposition of an AC circuit to current denoted by Z. In simple words, it gives the amount of circuit that impedes the flow of change. Impedance is like resistance, which also takes into account the effects of inductance and capacitance. The measurement unit for Impedance is ohms.
As impedance considers the effects of inductance and capacitance and varies with the frequency of current passing through the circuit, it is more complex than resistance. As compared to resistance, which is constant regardless of frequency, the impedance varies with frequency.
When it comes to defining reactance, it is the measure of the opposition of inductance and capacitance to current. Let’s learn more about these two terms in brief.
Impedance Formula
The mathematical symbol of impedance is Z, and the unit of measurement is the ohm. It is the superset of both resistance and reactance combined.
In phasor terms, impedance Z is characterized as a summation of resistance R and reactance X as:
X = R + j X
Where reactance X is the summation of iInductive reactance X\[_{L}\] and capacitive X\[_{C}\].
X = X\[_{L}\] - X\[_{C}\]
Impedance, Z = \[\frac{V}{I}\]
V = voltage in volts (V)
I = current in amps (A)
Z= impedance in ohms (Ω)
R= resistance in ohms (Ω)
Impedance can be split into two parts:
Resistance R (a part which is constant regardless of frequency)
Reactance X (a part that varies with frequency due to capacitance and inductance)
The alternating current lags or leads the voltage depending upon the nature of the reactance component of impedance (whether predominantly inductive or capacitance).
The inductance and capacitance cause phase shifts between current and voltage, which means the resistance and reactance cannot be simply summed up to give impedance. Instead, they must be summed up as vectors with reactance at right angles to resistance, as shown in the figure below.
Impedance Z = \[\sqrt{R^{2}+X^{2}}\]
There are four electrical quantities that determine the impedance (Z) of a circuit: These are resistance (R), capacitance (C), inductance (L), and frequency (f).
What is Reactance?
The measure of the opposition of inductance and capacitance to current in an AC circuit is known as reactance, denoted by X. It varies with the frequency of electrical signals and is measured in ohms.
Reactance is of two types:
Capacitive reactance ( X\[_{C}\] ), and
Inductive reactance ( X\[_{L}\] ).
Reactance Formula
The total reactance (X) is equal to the difference between the two:
Total reactance, X = X\[_{L}\]– X\[_{C}\]
a. Capacitive Reactance X\[_{C}\]
The reactance, which is large at low frequencies and small at high frequencies is known as capacitive reactance ( X\[_{C}\] ). X\[_{C}\] is infinite for steady DC, at zero frequency (f=0Hz). This means that the capacitor passes AC but blocks DC.
Capacitive Reactance, X\[_{C}\] = 1/2fC
Where,
X\[_{C}\] = reactance in ohms (Ω)
f = frequency in hertz (Hz)
C = capacitance in farads (F)
For example, 1µF capacitor has a reactance of 3.2k for a 50Hz signal, but when a frequency is higher at 10kHz, the reactance is only 16.
b. Inductive Reactance, X\[_{L}\]
The reactance which is small at low frequencies and large at high frequencies is known as inductive reactance. X\[_{L}\] is zero for steady DC, at zero frequency (f=0Hz). This means that the inductor passes DC but blocks AC.
The formula for calculating the inductive reactance of a coil is:
Inductive reactance, or X\[_{L}\] is a product of 2 times (pi), or 6.28, frequency of the ac current in hertz, and the inductance of the coil, in henries.
X\[_{L}\] = 2x f x L
L = the inductance value of coil in henries.
Inductive reactance, X\[_{L}\] = 2fL
Where,
X\[_{L}\] = reactance in ohms (Ω)
f = frequency in hertz (Hz)
L =inductance in henry (H)
For example, a 1mH inductor has a reactance of only 0.3 for a 50Hz signal, but when the frequency is higher at 10 kHz, its reactance is 63 .
Reactance and Impedance Formula
An element in the DC circuit can be easily described by using only its resistance. The resistance of a capacitor in DC circuits is regarded to be an open connection whereas the resistance of an inductor in a DC circuit will be regarded as a short connection or zero resistance. As opposed to DC circuits, it is seen that in AC circuits, the impedance of an element is a value of how much the element tends to oppose the flow of current when an AC voltage is being applied across it. Impedance can be represented as a complex number that consists of both real and imaginary parts and can be represented as follows:
Z = R + jX
Where Z represents the impedance
R represents the value of resistance
X which is the imaginary part will represent reactance
It is seen that the resistance seen in the circuit will always be positive while the reactance that is seen will be either positive or negative.
Do you know?
When current and voltage are out of step with each other, it means there is a phase shift. For example, when you charge a capacitor, the voltage across it is zero. However, the current is maximum. When the capacitor is charged, the voltage will be maximum, and the current will be at a minimum. The charging and discharging occur continually with AC, where the current reaches maximum shortly before the voltage reaches the maximum, so it is called current leading voltage.
FAQs on Reactance and Impedance
1. How do you calculate reactance from impedance?
To calculate impedance, calculate resistance and reactance of a circuit, label resistance as R and reactance as X. Square both R and X, and sum the two products together. Take the square root of the sum of the squares of R and X to get impedance. Display the answer in ohms.
Impedance Z = \[\frac{V}{I}\]
Z = √R² + X²
Z²= (R²+X²)
2. What is the difference between resistance, reactance, and impedance?
The measure of opposition to current flow offered by a material is known as resistance, denoted by R.
The resistance offered to AC currents only by inductors and capacitors is known as reactance, denoted by X.
For capacitor X= \[\frac{1}{\left (2\pi fC \right )}\] , where f is frequency, and C is capacitance.
For inductor X= 2πfL, where f is frequency, L is inductance.
Impedance is the summation of the resistance and reactance of a circuit.
Symbolized for primarily inductive circuit Z=R+jX or for primarily capacitive circuit Z=R−-jX.
Where j=√(-1).
3. When should I use impedance instead of reactance vice versa?
There is a phase shift between voltage and current, and iImpedance is sensitive to frequency.
Reluctance is a measure of the resistance of an ideal coil at a specific frequency.
Reactance is a measure of the resistance of an ideal capacitor at a specific frequency.
As an engineer:
Coils are imperfect, and the wire has an intrinsic resistance and loops form stay capacitance. Thus, a coil has resistance, reactance, and reluctance; this combination is a measure of its impedance.
Resistors are also imperfect, wire wound resistors have inductance to give small reluctance, and some surface mount resistors have tiny capacitance.
Normally speaking, if there is a DC circuit, you need to use resistance, and if the circuit is AC, you need impedance.
4. What is the difference between reactance and resistance?
The opposition of the inductor to change the current is usually termed as inductive reactance This is similar to resistance, but different in that it always results in phase shift between current and voltage and it disperse zero power. Due to the difference, it has a different name reactance. Just like resistance, reactance to AC is measured in ohms, but its mathematical symbol is X instead of R.
Some more differences between reactance and resistance are:
Parameter | Resistance | Reactance |
Variation in current | It is the property of the electrical component which opposes the flow of current. | It is the property of the electrical component which opposes the change in current. |
Power dissipation | It leads to power dissipation | It does not lead to power dissipation. |
Denoted by | It is denoted by R. | It is denoted by X. |
5. What are the types of reactance available?
There are mainly two types of reactance that are available and can be provided as follows:
1. Capacitive reactance:
When a capacitor is attached to the circuit with an AC supply, there is a simultaneous change in the capacitor voltage and capacitor current. The potential difference that has been applied across the capacitor is dependent on the AC power supply.
2. Inductive reactance:
The current that has been applied across the inductor changes when there is a potential difference being developed across it. The potential difference and the rate of change of current are directly proportional to each other in this case.
6. What is a wattless current?
The component of the AC current that consumes no power in a circuit will be called a wattless current. The equation of the average power that has been dissipated by the AC circuit can be provided as follows:
P = VI cos Φ
It is seen that the average power that has been dissipated in a circuit will not only depend on the voltage and current but it will also depend on the phase angle that is present between them. In a circuit where there is only the presence of a capacitor or inductor, the phase difference between the voltage and current will be π/2. Therefore, the cos Φ becomes zero and the power that has been dissipated will hence also become zero. There will be no power dissipation seen even though there is a current that is flowing through the circuit. This particular current is hence called the wattless current where no power dissipation takes place.
