How capaitor acts as a filter ?(reason behind its action)

Q1. How capaitor acts as a filter ?(reason behind its action)
Q2. Why capacitors with high capacitance are used at power rails and the one with low capacitance at power dips ?? ( i am asking how to decide which capacitor should be used where )
Q3. Why capacitor blocks dc current while allows ac current to pass? i know mathematically but want to know physically

amangupta009 said:

Q1. How capaitor acts as a filter ?(reason behind its action)

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It's many many possible ways a filter could be assembled. It's also possible to make a filter without a capacitor. Read through any chapter that describes RC filter to get the basic theory first.

amangupta009 said:

Q2. Why capacitors with high capacitance are used at power rails and the one with low capacitance at power dips ?? ( i am asking how to decide which capacitor should be used where )

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What kind of power rails? DC or AC? It's wasted time to explain this if you don't know any theory behind capasitors and drossels.

amangupta009 said:

Q3. Why capacitor blocks dc current while allows ac current to pass? i know mathematically but want to know physically

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Ask your teacher to do a lab on capacitors. Do some practical exsercises.

For Q1,

My suggestion: Study phasor domain

Also, I agree with Prototyp_V1.0. You should learn basics of circuit theory to fully understand answers of these questions.

Why capacitor blocks dc current while allows ac current to pass? i know mathematically but want to know physically

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That's sound promising, if it's true. There are many people out there, who don't understand capacitor operation mathematically.

To get a physical understanding, you should start with the basic design of a capacitor. In my opinion, it gives a graphic idea, why DC current can't pass the device. Considering the ability of a capacitor to store charges, you should be able to understand, why it conducts alternating current. For some background information, see also: Capacitor - Wikipedia, the free encyclopedia

P.S.: Also Chapter 13 Capacitors from , Volume I DC, gives a good explanation of the physical operation of capacitors

Hello, it´s easy to understand a capacitor like a special kind of "resistor" that have "variable resistance" with frequency. How much higher the signal frequency , lower the capacitor "special resistance".

Xc = (1/2πfC) (ohms) capacitive reactance ( capacitor "resistance" for AC signals)

Q3 Tthe physical properties of a capaditor is two plates isolated by an insulator. No current flows through either AC or DC. If you charge up one plate with electrons, the other plate will respond accordingly. With DC the plate charges up once and the other plate does the same.. only once until you remove the charge. With AC, one plate charges up and then discharges.. the other plate responds the same.. end result is no curent is really passed through the capacitor with AC.. but you get a push pull of charge / discharge effect on the oposite plate of the capacitor which can be measured as curent.. In DC circuits.. electrons flow through to pass energy. in AC circuits electrons push \ pull to pass energy.

For Q1:
You might have seen in many analog filter design books, that a RC combination can act as both low pass (see attached fig.1) and also as high pass (see attached fig.2).
For the lowpass configuration, a resistor allows low frequencies and the capacitor does not allow low frequencies (Xc = 1/wC), since at low frequencies high impedance is presented by the capacitor, so fig.1 allows only low frequencies. So only low frequency signals appear at the output.And for high frequencies, capacitor acts as short (low impedance) and they are grounded.

For the highpass configuration, capacitor allows high frequencies(Xc = 1/wC), as low impedance is presented by the capacitor at high frequencies, and high frequency signals appear across the resistor and at the output. And for low frequencies, capacitor acts as open (high impedance) and hence only high frequencies pass through the configuration shown in fig.2

For Q2:
Capacitors are used across power rails to reject the high frequency noise from the external sources, in other words to short the high frequency noise signals to ground. So if your capacitance value is high, the impedance is low (<10 ohm), and they acts as short to ground. Generally
0.01uF capacitors are used across power rails.