Electrical Impedance

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What is Impedance?

Impedance is one of the attributes of electronic components that measure resistance or opposition to the alternating current or direct electric current

Impedance is a vector quantity that consists of two independent one-dimensional phenomena, viz: resistance and reactance.

The symbol for impedance is Z. It is similar to that of the resistance, the formula for the impedance is as follows:

Z = V (in volts) / I (in amperes)

On this page, we will discuss the impedance definition and how is impedance measured.

What is Electrical Impedance?

Impedance reduces the opposition to the steady direct current ‘electric current’ flowing through the circuit. 

The magnitude of the impedance is equal to the potential difference applied across the circuit divided by the maximum current flowing through the circuit.

Bow, let’s consider a scenario to understand the ‘Impedance Definition’ most simply:

Electrical Impedance

Let’s suppose that you are in hurry for your tuition classes and your mother asks you to pour two gallons of milk into small cans and deliver these to your neighbours. 

Now, using a funnel would take a lot of time and you would get late for your classes. So, this time the potential difference applied by you is more, also, the milk flow too but the resistance offered is high and because of which you got late and got punishment from your teacher.

Another day, you chose a funnel with a big hole and it saved a lot of time. You did your job well and reached tuition class on time.

Here, the small funnel is the resistance, because of which the current flow or the milk flow to milk cans took time. However, a big funnel is an impedance that reduced opposition to the milk flow.

So, the impedance of the milk flow reduces time. 

As we discussed the electrical impedance with real-life scenarios, now, let’s understand what is impedance in Physics.

What is Impedance in Physics?

Electrical Impedance (Z) is the total opposition/resistance that a circuit offers to alternating current. 

Impedance is measured in ohms and may include resistance (R), inductive reactance (XL), and capacitive reactance (XC); the total impedance is the algebraic sum of the resistance, inductive reactance, and capacitive reactance. Since the inductive reactance and capacitive reactance are 90o out of phase with the resistance and therefore, their maximum values occur at varying times, we can use vector addition to calculate impedance.

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The relationship between impedance and its two components, viz: resistance and inductive reactance can be represented using a vector as shown in the image below:

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In the above image, the amplitude of the resistance component is represented by a vector along the x-axis & the amplitude of the inductive reactance is a vector along the y-axis. The amplitude of the impedance is shown by a vector that initiates from zero to a point that represents both the resistance value in the x-direction and the inductive reactance in the y-direction. 

The electrical impedance in an electrical circuit with resistance and inductive reactance can be calculated by using the following equation. If capacitive reactance is present in the circuit, its value is added to the inductive reactance term before squaring.

So, the equation for the above statement is given by:

Z = \[\sqrt{Xl^{2}+R^{2}}\]

Impedance & Ohm’s Law

In the above text, Ohm's Law is stated for a purely resistive circuit. When an inductive reactance or capacitive reactance present in the circuit, Ohm's Law can be written to include the total impedance in the circuit. Therefore, Ohm's law takes the following form:

I = V / Z

In the above equation, Ohm's law simply states that the current (I), in amperes, is proportional to the voltage (V), in volts, divided by the impedance (Z), in ohms.

Please note that the values of resistance or the inductive reactance must be changed to vary the impedance in the circuit.

Impedance vs Resistance

Impedance is just like resistance. It is a value that shows the amount of resistance that a component offers to the flow of electrical current. And just like resistance, the unit of impedance is Ohms (Ω).

However, unlike resistance, the impedance varies with the amount of resistance that a component has to a signal varies on the frequency of the signal. This means that the resistance of the component varies directly with the frequency of the signal entering the electronic component. 

Resistance is a value and its measure is independent of frequency; also, it doesn't take into account the frequency of the signal passing through it, because frequency doesn't affect the resistance of non-reactive components. However, reactive components change the amount of resistance they offer in a circuit relies on the input signal's frequency. But impedance varies with the frequency of the signal passing via it. So, this was the difference between resistance and impedance.

FAQ (Frequently Asked Questions)

1. What do you mean by inductive reactance?

In an electronic circuit, inductive reactance aka reactance is the resistance in an inductive circuit. It slightly varies from the term resistance. 

The effect under which the flow of an alternating or changing current in an inductor reduces is called its inductive reactance. Any changing current in an inductor impedes as a result of the inductance linked with it.

So, if a changing signal such as a sine wave is applied to an inductor, the reactance impedes/delays the flow of current and follows Ohm’s law. The inductive reactance of an inductor relies upon its inductance and the frequency applied. The reactance increases linearly with the frequency of the signal.

2. How to calculate the impedance of the circuit?

To calculate the impedance of the circuit, we determine the two following impedances:

  1. Capacitor impedance

  2. Inductor impedance

To calculate the impedance, let’s look at the two following approaches:

1. Capacitor Impedance

XC = 1/2𝜋fC

Where XC is the impedance in-unit ohms

f = The frequency of the signal passing through the inductor, and 

C = capacitance of the capacitor 

2. Inductor Impedance

XC = 2𝜋fL


L is the inductance of the inductor.

If there are both capacitors and inductors reside in a circuit, we can calculate the total amount of impedance by adding all of the individual impedances in the following manner:

XTotal = XC + XL