Lesson Explainer: Impedance of Alternating Current Circuits Physics

In this explainer, we will learn how to calculate the impedance of simple resistive-capacitive-inductive circuits, using capacitive and inductive reactances.

Reactance has a specific, precisely defined meaning and is defined for circuits that are connected to sources of alternating potential difference.

Before considering the exact meaning of reactance, we can first say that it is a property that can be compared to resistance, but with significant differences from it.

The way in which reactance is like resistance is that they both act to reduce current in a circuit.

An inductor with inductance reacts to a change in current in a time interval by generating an opposing emf, , where

The induced opposing emf acts to reduce the instantaneous current through the inductor. The reduced instantaneous current produces a smaller opposing emf and so a smaller opposing current. The opposing current decreases with time.

To show how inductance relates to reactance, we must consider a circuit containing an inductor that is connected to a continuously varying source of potential difference, such as a generator that supplies an alternating emf at some frequency .

Whether or not such a circuit has inductance, the current in the circuit varies sinusoidally.

For such a circuit, we can define its inductive reactance, . We can call the maximum value of current in the circuit .

Recall that

It is the case that

We can see this proportionality demonstrated in the following graph of two sinusoidal currents with the same frequency.

The greater the inductance of an inductor is, the more slowly the current in the inductor can change.

We see then that the greater the value of , the less the value of .

It then follows that for a given value of , the greater the value of is, the less the maximum value of must be.

It is also important to note that the frequency of a sinusoidally alternating current must determine the rate of change of that current.

It must also then be the case that

The following graph shows three sinusoidal currents.

We see that the maximum value of current is the same as the maximum value of current .

We see also that the frequency of is greater than the frequency of ; hence, the value of of at is greater than the value of of at .

Current has the same frequency as current but a lower maximum value.

At , current has the same value of as current .

This inequality of the maximum values of and is necessary for to have the same value of as at and to also have the same frequency as .

We see then that the greater the value of , the less the maximum value of .

Considering the dependence of on , , and , we see that

The value of is defined as the ratio of to .

We see from this that

Reactance does not have a direction, and so the negative sign is irrelevant, and hence it can just as well be said that

A formula for inductive reactance can be derived from these relationships.

We see that inductive reactance has the same unit as resistance. Inductive reactance is effectively a resistance that only applies to alternating current circuits containing inductors.

We see that increasing the inductance of a circuit increases its inductive reactance. We see also that increasing the frequency of an alternating current increases the inductive reactance of the circuit.

Let us look at an example involving inductive reactance.

As well as resistors and inductors, a circuit can also contain capacitors. Capacitors also have reactance, called capacitive reactance, .

Before defining capacitive reactance, let us consider how a capacitance acts to reduce the average current in a circuit.

Let us consider a circuit containing a resistor and a capacitor in series.

A capacitor can store charge that is deposited on the capacitor by the current in a circuit that contains the capacitor. The maximum charge, , that can be stored by a capacitor with capacitance that is connected to a potential difference is given by

A capacitor initially stores zero charge and hence has zero potential difference across it. The more the charge stored by a capacitor, the greater the potential difference across it.

The more charge that is stored by the capacitor, the more the current in the circuit is reduced. When a capacitor is storing the maximum charge that it can store, the current in the circuit containing the capacitor must be zero.

The relationship between the potential difference across a capacitor and the current in a circuit containing a capacitor can be expressed as

If a capacitor has zero capacitance, when connected to a circuit, it is initially storing its maximum charge, and so the current in the circuit must be zero. The greater the capacitance of the capacitor is, the more the charge that can be transported by the current in the circuit before the current becomes zero.

We can see from this that when a circuit containing a capacitor is connected to a potential difference source over a certain time interval, the current in the circuit will be reduced less if a capacitor of greater capacitance is used.

What we see from this is that a capacitor can also act to reduce the current in a circuit, similarly to the way that an inductor can. However, the greater the capacitance of the capacitor, the less the current is reduced.

Capacitive reactance has a specific, precisely defined meaning.

To show how capacitance relates to capacitive reactance, we must consider a circuit containing a resistor and a capacitor in series that is connected to a continuously varying source of potential difference, such as a generator that supplies an alternating emf at some frequency .

Whether or not such a circuit has capacitance, the current in the circuit varies sinusoidally.

For such a circuit, we can define its capacitive reactance, . We can call the maximum value of the potential difference across the capacitor .

Recall that

It is the case that

It is also the case that

We see then that

The value of is defined as the ratio of to .

We see from this that

A formula for capacitive reactance can be derived from these relationships.

We see that capacitive reactance has the same unit as resistance. Capacitive reactance is effectively a resistance that only applies to alternating current circuits containing capacitors.

We see that increasing the capacitance of a circuit decreases its capacitive reactance. We see also that increasing the frequency of an alternating current decreases the capacitive reactance of the circuit.

It is useful to note that the relationship of capacitance and of frequency to capacitive reactance is the inverse of the relationship of inductance and of frequency to inductive reactance.

Let us look at an example involving capacitive reactance.

The current in an alternating current circuit at a given instant depends on the combined effect of the resistance and the reactance of the circuit. This is called the impedance, , of the circuit.

A circuit containing a resistor (R), inductor (L), and capacitor (C) connected to an alternating potential difference source is shown in the following figure.

The resistor provides the circuit with resistance. The inductor and capacitor provide the circuit with reactance.

For any alternating current circuit, the potential difference, , across the circuit varies with time. We can denote by the potential difference across the circuit at the instant .

If such a circuit has resistance, , but no reactance, the current at a given instant, , is determined as it would be in a direct current circuit, using Ohm’s law. We can express this as

However, if an alternating current circuit has a reactance, the instantaneous current in the circuit is affected by the reactance of the circuit. The instantaneous current in such a circuit is given by

The capacitive and inductive reactances are not simply summed to determine their contribution to the impedance.

The instantaneous current passed by a capacitor is not in phase with the instantaneous potential difference across the capacitor. The current leads the potential difference by a phase angle of .

The instantaneous current passed by an inductor is not in phase with the instantaneous potential difference across the inductor. The current lags the potential difference by a phase angle of .

These phase relationships are shown in the following figure.

The phase difference between current and potential difference is for both capacitors and inductors, but in opposite directions.

We can see then that the phase difference between the current and the potential difference for a capacitor compared to an inductor is , which is .

This means that the effect on the instantaneous current of a capacitor and that of an inductor are opposite to each other.

From this, we can see that if the capacitive and inductive reactances of a circuit are equal, the effect on the current due to the total reactance is zero. In this case, the impedance of the circuit is just the resistance of the circuit.

From these relationships, a formula for impedance can be obtained.

It is important to notice that a circuit can have nonzero impedance despite having zero resistance. For an alternating current circuit, the reactance of the circuit alone can constrain the value of current. This is not the case for circuits supplied by a constant potential difference.

Let us look at an example involving impedance.

Let us look at another such example.

Let us look at an example in which currents are determined.

Let us now summarize what has been learned in this explainer.