Don't be scared by complex numbers. Just do everything as if they are real numbers. Then, when we want to extract the information from the calculation, just consider real part.
Suppose the voltage from generator is
. This voltage goes across internal impedance (let's call in
) and passive load (
) in series.
The current in the circuit is now just
.
Power to the load is just
Now,
. To avoid large amount of calculation, let's use hand-waving argument. In order to keep the norm of the denominator small (so that the power is large), the imaginary part should go to zero. Otherwise, it will introduce some phase and larger denominator. This sounds like cheating since we haven't taken account of
at numerator. However, the denominator has power of two, while the numerator has only one. So, it's OK.
Hence, to make imaginary part of
be zero,
The correct answer is (C). Only 22 of 100 people answer correctly.
ฟิสิกส์โอลิมปิก วิทยาศาสตร์โอลิมปิก ข้อสอบแข่งขัน ข้อสอบชิงทุน
