How Does Impedance Work?

Impedance is the total amount of opposition to the flow of current in a circuit. Impedance works due to the actions of inductive reactance, capacitive reactance and resistance on the circuit, which combine to resist the flow of current. Ohm () is used for measuring impedance.

Since the impact of capacitive reactance and inductive reactance change with the frequency of the current flowing across the circuit, how impedance works is a lot more complicated than how resistance works. While resistance remains constant even with changes in frequency, impedance tends to vary.

Impedance (Z) = V/I 

There are two components to impedance:

1. Resistance: This remains constant with a change in frequency.
2. Reactance: This is due to capacitance and inductance and changes with frequency.

In circuits without capacitance or inductance, impedance is the same as resistance.

How Does Impedance Work?

As we know, impedance is caused by resistance and reactance. Let’s see what causes these properties and how impedance works as a result.

What Causes Resistance?

Resistance works at the molecular level, opposing the flow of current. Consider a metal conductor like aluminium. The outermost shells of aluminium atoms contain free electrons. There is a random movement of free electrons between atoms.

However, when a device such as a battery is used to apply a voltage across the conductor, the free electrons start moving from the negative terminal to the positive terminal of the battery. The free electrons start flowing due to the electric current, which is nothing but the rate of flow of electric charge.

The electrons which move in the direction of the positive terminal clash with other electrons, atoms and impurities in the metal. This is due to the imperfections in the atomic lattice of metals, which causes the scattering of electrons. The resistance in the conductor works due to these collisions. Heat is generated from the voltage supplied as the electrons clash with atoms and other particles.

What Causes Reactance?

Reactance is the resistance exhibited by the movement of alternating current in an inductor or capacitor due to inductance or capacitance. It plays a key role in how impedance works. In a few ways, reactance is like an alternating current equivalent of direct current resistance.

While resistance disperses energy as heat, when there is reactance to the flow of alternating current in an inductor, capacitor or conductor, the energy is stored and released from a magnetic or an electric field. 

There are two types of reactance. The reactance is inductive if the field is magnetic, i.e. when the circuit contains an inductor. When there is an electric field, the reactance is capacitive, i.e., when the circuit contains a capacitor. 

Reactance is of two types: capacitive and inductive.

Inductive Reactance

When there is resistance to the movement of alternating current in an inductor, it is known as inductive reactance. 

The current in a purely inductive AC circuit lags the applied voltage by 90° or (π/2 rads). This is because the applied voltage and the current cannot remain in phase due to the opposing force of the counter emf. 

This opposing force is the cause of inductive reactance.

The inductive reactance is directly proportional to the frequency of the current and the inductance. Therefore, the formula of inductive reactance is X_L = ωL, where ω is the frequency and L is the self-impedance of the inductor. 

Capacitive Reactance

When there is resistance to the movement of alternating current in a capacitor, it is known as capacitive reactance. The current in a purely capacitive AC circuit is ahead of the voltage across the capacitor by 90°or π/2 rads. This is due to the charging and discharging properties of the capacitor. 

The capacitive reactance is inversely proportional to the frequency of the current and the capacitance. Therefore, the formula of capacitive reactance is X_c = 1/ωC. Where ω is the frequency and C is the self-capacitance of the inductor. 

The reactance of the capacitor decreases as the frequency increases and the reactance increases as the frequency decreases.

How Is Impedance Measured?

The simple addition of reactance and resistance does not give impedance. This is because of the shift in phase in the voltage and current due to inductance and capacitance. The reactances and resistance, therefore, undergo vectorial addition at right angles to measure impedance. 

To find the value of the impedance (Z): Reactance (X) and resistance (R) are represented in a right-angled triangle. Resistance is considered as the base, with impedance as the hypotenuse and reactance at right angles to resistance.

Using the Pythagoras theorem, 

we see that  Z² = X²+R².    

What Are The Uses Of Impedance?

The use of impedance can be found everywhere. It can be seen in a fuse panel that controls the flow of electricity in a building. It is used to limit the count of electrical mishaps. When there is a surge in the current, the fuse breaks the flow of current, reducing the count of electrical mishaps. 

Impedance can also be used in capacitors. Impedance works to regulate the flow of electricity on a circuit board. Electricity is delivered in varying pulses in an alternating-current circuit. The capacitor performs the function of storing electricity and releasing it smoothly to ensure that there is no power surge or low current. Without capacitors controlling the flow of electricity, there could be potential damage to appliances. 

Conclusion

Impedance is the full amount of resistance to the flow of current in a circuit. Impedance works due to the actions of inductive reactance, capacitive reactance and resistance on the circuit, which combine to resist the flow of current. While resistance remains constant even with changes in frequency, impedance tends to vary due to the impact of capacitive and inductive reactance. The simple addition of reactance and resistance does not give impedance. This is because of the shift in phase in the voltage and current due to the inductance and capacitance. It is given by Z² = X²+ R². Impedance is useful when it comes to protecting circuits from damage.