Download Article
Download Article
Impedance is the opposition of a circuit to alternating current. It's measured in ohms. To calculate impedance, you must know the value of all resistors and the impedance of all inductors and capacitors, which offer varying amounts of opposition to the current depending on how the current is changing in strength, speed, and direction. You can calculate impedance using a simple mathematical formula.
Formula Cheatsheet
- Impedance Z = R or X L or X C (if only one is present)
- Impedance in series only Z = √(R 2 + X 2 ) (if both R and one type of X are present)
- Impedance in series only Z = √(R 2 + (|X L - X C |) 2 ) (if R, X L , and X C are all present)
- Impedance in any circuit = R + jX (j is the imaginary number √(-1))
- Resistance R = ΔV / I
- Inductive reactance X L = 2πƒL = ωL
- Capacative reactance X C = 1 / 2πƒC = 1 / ωC
Steps
-
Define impedance. Impedance is represented with the symbol Z and measured in Ohms (Ω). You can measure the impedance of any electrical circuit or component. The result will tell you how much the circuit resists the flow of electrons (the current). There are two different effects that slow the current, both of which contribute to the impedance: [1] X Research source
- Resistance (R) is the slowing of current due to effects of the material and shape of the component. This effect is largest in resistors , but all components have at least a little resistance.
- Reactance (X) is the slowing of current due to electric and magnetic fields opposing changes in the current or voltage. This is most significant for capacitors and inductors .
-
Review resistance. Resistance is a fundamental concept in the study of electricity. You'll see it most often in Ohm's law : ΔV = I * R. [2] X Research source This equation lets you calculate any of these values if you know the other two. For instance, to calculate resistance, write the formula as R = ΔV / I . You can also measure resistance easily, using a multimeter .
- ΔV is the voltage, measured in Volts (V). It is also called the potential difference.
- I is the current, measured in Amperes (A).
- R is the resistance, measured in Ohms (Ω).
Advertisement -
Know which type of reactance to calculate. Reactance only occurs in AC circuits (alternating current). Like resistance, it is measured in Ohms (Ω). There are two types of reactance, which occur in different electrical components:
- Inductive reactance X L is produced by inductors, also called coils or reactors. These components create a magnetic field that opposes the directional changes in an AC circuit. [3] X Research source The faster the direction changes, the greater the inductive reactance.
- Capacitive reactance X C is produced by capacitors, which store an electrical charge. As current flows in an AC circuit changes direction, the capacitor charge and discharges repeatedly. The more time the capacitor has to charge, the more it opposes the current. [4] X Research source Because of this, the faster the direction changes, the lower the capacitive reactance.
-
Calculate inductive reactance. As described above, inductive reactance increases with the rate of change in the current direction, or the frequency of the circuit. This frequency is represented by the symbol ƒ, and is measured in Hertz (Hz). The full formula for calculating inductive reactance is X L = 2πƒL , where L is the inductance measured in Henries (H). [5] X Research source
- The inductance L depends on the characteristics of the inductor, such as the number of its coils. [6] X Research source It is possible to measure the inductance directly as well.
- If you're familiar with the unit circle , picture an AC current represented with this circle, with one full rotation of 2π radians representing one cycle. If you multiply this by ƒ measured in Hertz (units per second), you get a result in radians per second. This is the circuit's angular velocity , and can be written as a lower-case omega ω. You might see the formula for inductive reactance written as X L =ωL [7] X Research source
-
Calculate capacitive reactance. This formula is similar to the formula for inductive reactance, except capacitive reactance is inversely proportional to the frequency. Capacitive reactance X C = 1 / 2πƒC . [8] X Research source C is the capacitance of the capacitor, measured in Farads (F).
- You can measure capacitance using a multimeter and some basic calculations.
- As explained above, this can be written as 1 / ωC .
Advertisement
-
Add resistances in the same circuit. Total impedance is simple if the circuit has several resistors, but no inductors or capacitors. First, measure the resistance across each resistor (or any component with resistance), or refer to the circuit diagram for the labeled resistance in ohms (Ω). Combine these according to how the components are connected: [9] X Research source
- Resistors in series (connected end to end along one wire) can be added together. The total resistance R = R 1 + R 2 + R 3 ...
- Resistors in parallel (each on a different wire that connects to the same circuit) are added as their reciprocals. To find the total resistance R, solve the equation 1 / R = 1 / R 1 + 1 / R 2 + 1 / R 3 ...
-
Add similar reactance values in the same circuit. If there are only inductors in the circuit, or only capacitors, the total impedance is the same as the total reactance. Calculate it as follows: [10] X Research source
- Inductors in series: X total = X L1 + X L2 + ...
- Capacitors in series: C total = X C1 + X C2 + ...
- Inductors in parallel: X total = 1 / (1/X L1 + 1/X L2 ...)
- Capacitors in parallel: C total = 1 / (1/X C1 + 1/X C2 ...)
-
Subtract inductive and capacitive reactance to get total reactance. Because one of these effects increases as the other decreases, these tend to cancel each other out. To find the total effect, subtract the smaller one from the larger. [11] X Research source
- You will get the same result from the formula X total = |X C - X L |
-
Calculate impedance from resistance and reactance in series. You can't just add the two together, because the two values are "out of phase." This means that both values change over time as part of the AC cycle, but reach their peaks at different times. [12] X Research source Fortunately, if all of the components are in series (i.e. there is only one wire), we can use the simple formula Z = √(R 2 + X 2 ) . [13] X Research source
-
Calculate impedance from resistance and reactance in parallel. This is actually a general way to express impedance, but it requires an understanding of complex numbers. This is the only way to calculate the total impedance of a circuit in parallel that includes both resistance and reactance.
- Z = R + jX, where j is the imaginary component: √(-1). Use j instead of i to avoid confusion with I for current.
- You cannot combine the two numbers. For example, an impedance might be expressed as 60Ω + j120Ω.
- If you have two circuits like this one in series, you can add the real and imaginary components together separately. For example, if Z 1 = 60Ω + j120Ω and is in series with a resistor with Z 2 = 20Ω, then Z total = 80Ω + j120Ω.
Advertisement
Calculator, Practice Problems, and Answers
Community Q&A
Search
-
QuestionWhat does the j stand for in an impedance calculation?This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness.wikiHow Staff EditorStaff AnswerThe j stands in for the imaginary number √(-1). Normally you would use i for this, but j is used in impedance calculations to avoid confusion with I for current.
-
QuestionHow do I calculate line impedance?This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness.wikiHow Staff EditorStaff AnswerLine impedance is the ratio of complex line voltage to complex line current. You can calculate it with the following equation: Z(z) = V(z)/I(z).
-
QuestionHow do I calculate the impedance of the coil connected in series with the capacitor?Community AnswerIf the coil has a resistance, then treat it as an LCR circuit. Z = [(Xc-XL)^2 + R]^0.5, where Z is the impedence, Xc is reactance of the capacitor which is equal to 1/(wC), w being the angular frequency and C being capacitance. XL is reactance of the coil which is equal to wL, L being the inductance of the coil, and R is resistance of the coil. Put R = 0 in case the coil has no resistance.
Ask a Question
200 characters left
Include your email address to get a message when this question is answered.
Submit
Advertisement
Video
Tips
- Total impedance (resistance and reactance) can also be expressed as a complexThanks
Submit a Tip
All tip submissions are carefully reviewed before being published
Name
Please provide your name and last initial
Thanks for submitting a tip for review!
Advertisement
References
- ↑ https://www.allaboutcircuits.com/textbook/alternating-current/chpt-5/review-of-r-x-and-z/
- ↑ https://www.physicsclassroom.com/class/circuits/Lesson-3/Ohm-s-Law
- ↑ https://www.electronics-tutorials.ws/accircuits/ac-inductance.html
- ↑ https://www.learnabout-electronics.org/ac_theory/reactance62.php
- ↑ https://www.learnabout-electronics.org/ac_theory/reactance61.php
- ↑ https://www.learnabout-electronics.org/ac_theory/inductors02.php#backemf
- ↑ https://www.allaboutcircuits.com/textbook/alternating-current/chpt-3/ac-inductor-circuits/
- ↑ https://www.learnabout-electronics.org/ac_theory/reactance62.php
- ↑ https://www.omnicalculator.com/physics/ohms-law-resistance
- ↑ https://www.omnicalculator.com/physics/capacitive-reactance
- ↑ https://www.omnicalculator.com/physics/capacitive-reactance
- ↑ https://www.allaboutcircuits.com/textbook/alternating-current/chpt-5/review-of-r-x-and-z/
- ↑ https://www.allaboutcircuits.com/textbook/alternating-current/chpt-3/series-resistor-inductor-circuits/
- ↑ https://www.electronics-tutorials.ws/accircuits/ac-inductance.html
- ↑ https://learnabout-electronics.org/ac_theory/impedance71.php
About This Article
Article Summary
X
To calculate impedance, calculate the resistance and reactance of the circuit, then label resistance as R and reactance as X. Square both R and X, and add 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. To learn how to calculate resistance and reactance, read on!
Did this summary help you?
Thanks to all authors for creating a page that has been read 936,308 times.
Reader Success Stories
- "The article has helped me learn the difference between resistance, reactance, and impedance. I had problems differentiating there terms. Thanks, wikiHow. Now I can write my lab report without difficulties." ..." more
Advertisement
83%
of readers found this article helpful
.
Click a star to add your vote
% of people told us that this article helped them.
"The article has helped me learn the difference between resistance, reactance, and impedance. I had problems differentiating there terms. Thanks, wikiHow. Now I can write my lab report without difficulties."
..."
more
"Just getting a good foundation and explanation of the formulas was a wonderful start. Unfortunately, it did not delve into parallel circuits or series-parallel circuits."
..."
more
"I'm preparing to take the exam for the extra class Ham license & this article is helping me with some of the circuit-related questions."
..."
more
"This article is very good. It's helped me to understand AC circuit and its calculation. I love wikiHow very much."
..."
more
"I can build on the concepts shown here to learn more for my upcoming exam in engineering physics."
