In a series R-C circuit, a resistor (R) and a capacitor (C) are connected in series with each other. The behavior of this circuit depends on the frequency of the input signal.

At very low frequencies, the capacitor behaves as an open circuit, and the circuit behaves almost purely resistive. This means that the current through the circuit is determined by the resistance alone, and there is very little effect of the capacitor. The voltage across the resistor and the capacitor is in phase with each other, and the phase angle between the current and the voltage is close to zero.

At very high frequencies, the capacitor behaves as a short circuit, and the circuit behaves almost purely inductive. This means that the current through the circuit is determined by the reactance of the capacitor, and there is very little effect of the resistance. The voltage across the resistor and the capacitor is out of phase with each other, and the phase angle between the current and the voltage is close to 90 degrees.

This behavior can be explained by the impedance of the R-C circuit. The impedance is the complex counterpart of resistance and is given by Z = R + jX, where R is the resistance and X is the reactance. The reactance of the capacitor is given by Xc = 1/(2πfC), where f is the frequency of the input signal and C is the capacitance.

At low frequencies, the reactance of the capacitor is very large compared to the resistance, and hence the impedance is dominated by the resistance. At high frequencies, the reactance of the capacitor is very small compared to the resistance, and hence the impedance is dominated by the reactance.

In conclusion, at very low frequencies and very high frequencies, a series R-C circuit behaves almost purely resistive and inductive, respectively.