Sorry if this is a stupid question, but I'm in multiple electronics courses and I can't seem to understand input vs output impedance no matter who I ask.
I can always find the impedance between two points, if I am given the two points and I know all resistor/capacitor/inductor etc. values between those points along all possible paths between them. But when I'm asked to find input or output impedance of an entire circuit, I have no idea what those two points are. My professor seems to automatically understand which two points happen to correspond to input or output impedance in a given example, but to me, it seems completely arbitrary.
I understand that power supplies and loads have effective impedances, but often when I look at a circuit, I have no idea where the power supply ends and the load begins, or whether it's even relevant to think in terms of power supplies and loads... to me it's just a bunch of components jumbled together.
Here's some examples of worked homework problems (solutions in red):
Why is it that the input impedance of the circuit in part "a" is 10k? Obviously there's a 10k resistor on the op amp input, which seems extremely simple, but it's also extremely vague. Why don't I need to worry about the 500k resistor? Why should I care that the input has a 10k resistor when the signal also runs into a 500k resistor and an op amp with an enormous resistance?
Then in part "c" we add another op amp to greatly increase the input resistance. Now all of a sudden we do care about the large op amp resistance, just because we put one op amp in front of another.
It really seems to be that simple, but I feel like I'm just saying "I'll just throw a 10k resistor in front of this circuit and voila, there's you 10k input resistance, have a nice day." If it's really that simple, I at least want to know why it's a helpful idea.
Could someone please break down this concept and really explain it to me like I have never seen electricity in my life. For some reason it's so obvious to others, that they do a terrible job of understanding just how confused I am, and their explanation won't help.