Final answer:
The load impedance that maximizes power transfer in an AC circuit is the conjugate of the source impedance. At resonance, an RLC circuit is purely resistive and the power factor is 1, leading to maximal power transfer with impedance equal to resistance.
Step-by-step explanation:
In an AC circuit, the condition for maximum power transfer is achieved when the load impedance (Zl) is equal to the conjugate of the source impedance (Zs*). This is represented by the choice B) The conjugate of the source impedance (Zs*).
Maximum power transfer in AC circuits is fundamentally different from DC circuits due to the presence of a phase angle between the voltage and current. The concept of power factor, denoted as cos φ, plays a pivotal role, with values ranging from 0 to 1, and represents the ratio of the real power flowing to the load to the apparent power in the AC circuit.
At resonance, an RLC series circuit behaves like a purely resistive circuit leading to a power factor of 1, with the emf and current in phase, thereby maximizing power transfer to the load. The impedance (Z) of the circuit is simply the resistance (R) at resonance, because the inductive reactance (XL) and capacitive reactance (Xc) cancel each other out. Thus, the power at resonance is given by Pave = Vs / R, which is the equation for maximum power in an RLC circuit at resonance.