It turns out the capacitor blocked only very low frequency signals, between 0 Hz to about 0.5Hz, or 500 mHz. It will attenuate signals a little from about 0.5Hz to 3Hz. But after that, it no longer attenuates signals above 3Hz. Signals 4Hz and above go through completely unimpeded, unblocked and unattenuated.
With low frequency signals, little current flows in the capacitor, little voltage drop across the resistor, so most of the low frequency signal voltage appears on the capacitor. As you can see, filtering has already happened at that capacitor node, large low signal voltage with respect to ground, small high frqeuency voltage. Oh!
While in AC the capacitor is subjected to changing polarities and thus it keeps on charging/discharging according to the AC supply frequency. My question is why is it that capacitor blocks AC at low frequencies, since it also is AC only and is changing polarities at a lower rate so a capacitor can respond by charging/ discharging at low rate?
@BigBear The presence of that capacitor causes the high frequency current to flow to ground. That current causes a large voltage drop in the resistor feeding it, the voltage of the high frequency signal on that capacitor node is therefore very low.
Capacitors can be low pass high pass filters because their impedance changes with the frequency of the input signal. If we create a voltage divider of 1 stable impedance element (resistor) and 1 variable impedance element (capacitor) we can filter out low frequency or high frequency input signals.
If the frequency is high enough, the capacitor will barely charge/discharge, and most of the input voltage will be seen at the load, as if the capacitor didn't exist and was replaced with a wire. It basically let's through high frequencies.