Final answer:
Impedance in an AC circuit is composed of resistance (R), inductive reactance (XL), and capacitive reactance (XC), which together determine the opposition to current flow. These three components do not sum straightforwardly due to their differing effects on the AC circuit's characteristics.
Step-by-step explanation:
The three types of opposition to current flow that impedance represents are resistance, inductive reactance, and capacitive reactance. Resistance, denoted by R, is the opposition to current flow due to collisions of moving charges with atoms, which is present in both AC and DC circuits. Inductive reactance, denoted by XL, is the opposition of an inductor to a change in current, occurring only in AC circuits. Capacitive reactance, denoted by XC, is the opposition of a capacitor to a change in current, also exclusive to AC circuits. When these components are found together in an AC circuit, such as an RLC circuit, they do not simply add together because capacitors and inductors have opposite effects on the phase angle between voltage and current. Instead, the combination of these three components contributes to the total impedance (Z) of the circuit, which is calculated differently from simply summing resistance values in a DC circuit. Impedance plays a crucial role in the functioning of the circuit, as a higher impedance results in a lower current for a given voltage, according to an AC version of Ohm's law.