RC low pass filter difference with RC high pass filter

Question

This is a low pass filter:

^{simulate this circuit – Schematic created using CircuitLab}

This a high pass filter:

^{simulate this circuit}

If we add a load resistor to the high pass filter it becomes like this:

^{simulate this circuit}

The total resistance of the R1 with the RL will be lower than the smallest resistance(R1 or RL).

If we add a load resistor to the low pass filter it becomes like this:

^{simulate this circuit}

The total impedance of the C1 with the RL will be lower than the smallest impedance(C1 or RL).

If we start from a frequency when the impendance of C1 is smaller than the impendance of RL then the strength of the signal passing through RL will depend on frequency.

However if the impendance of the load resistor becomes less than the impedance of the capacitor then the strength of the signal passing through RL will not depend anymore on frequency

(in both cases i am reffering to voltage not current passing through RL resistor)

Low pass filters stop working below below a certain frequency and high pass filter always work?

Answer 1

“Low pass” means below a certain frequency, the filter will not change the input to output transfer. I guess you could say it “quits working” below a certain frequency, but I would not use this phrase.

“High pass” means below a certain frequency, it changes the input to output transfer ( making it smaller). So, I guess you could say, by definition, a high pass filter always “works”, ( that is, changes the transfer) at low frequencies.

So, if you use your definition of the word “works”, you are right! Most people don’t think of it this way, though.