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# Series Resonant Circuits

So far, we have merely considered AC circuits containing reactances of a *single* kind (capacitor or coil). Combining both these types of reactance results in an *oscillating circuit*. Due to their selective frequency properties they are usually called *resonant circuits.*

An resonant circuit includes both an inductance and a capacitance. |

Oscillations are produced through periodic build-up and decay of an electric field (capacitance) or magnetic field (inductance). The following animation illustrates this.

The circuit diagram below shows a *series resonant circuit* consisting of a capacitor, inductor and an active resistance. The latter might also represent, for instance, the ohmic resistance of the coil .

Shown below is a vector diagram of voltages in this circuit. The voltages *U*_{C} and *U*_{L} across the capacitor and inductor are effectively in opposite directions.

For this reason, the total reactance of a series circuit is the *difference* between the inductive and capacitive reactance. The series circuit's apparent resistance is therefore as follows: