Does a capacitor block DC or AC or both?

Answer 1

Current through a capacitor is proportional to the rate of change of voltage across it. But in the case of dc supply, the rate of change of voltage is zero. Hence the capacitor current is also zero.

Capacitive reactance -- the "resistance" of a capacitor to current flow -- is found by 1/(2*pi*f*C). For high frequencies, this "resistance" is low, allowing current flow. The lower the frequency, the higher this "resistance" is. DC or direct current is consider "zero" Hertz frequency and the capacitive reactance ("resistance") at this point is infinite, allowing no current flow at all. So, the higher the frequency, the easier it is for the signal to pass through the capacitor; the lower the frequency, the more difficult it is for current to pass and impossible for DC.

Answer 2

A capacitor blocks DC. Though the capacitor (called a cap) blocks direct current (DC) and will pass AC, it will not do so "evenly" across the spectrum of alternating current (AC). Let's look at that. When an AC signal "hits" a cap, it charges one plate while forcing charge off the other plate. This will give the cap an instantaneous polarity. As the polarity of the circuit reverses, as it does with AC, the charge on the cap will reverse. This is the mechanism by which the cap is able to "pass" AC. Additionally, the cap charges and discharges "out of phase" with the applied voltage in the circuit. Voltage across it will lag the applied voltage. One more thing and we're done. The higher frequency of the AC signal, the more easily that signal will pass through the capacitor. The "resistance" of a capacitor, called capacitive reactive, decreases as the frequency of the AC signal increases. There is an inverse relationship there. As a single example, in a coaxial signal delivery system like cable TV, the higher the frequency of the signal, the more loss there is to that signal through the cable system. That's because of the distributed capacitance in the coax. We'll check that out and sign off. That capacitance in coax cable is distributed down the length of the cable, and it appears across the space between the center conductor and the shield. Higher frequency signals are able to more effectively "jump the gap" and get across the insulating material in the coax than lower frequency signals. That's why there is more loss of the higher frequency signals in the network. It creates a condition called "slope" in the system, and that's where the higher frequency signals get to the "far end" of the cable and appear at lower levels than the lower frequency signals.

Answer 3

A coil doesn't block either AC or DC; It does offer a resistance to electrical current and is often used to filter signal components to levels where they are insignificant. It is a small but significant distinction. Here is how it works (With apologies to the electronics purists and academics): A coil has two resistive effects. The DC resistance of wire is one of them. All coils have this resistance but it is normally very low. The coil also has inductance: This is the resistance to AC current. The higher the frequency of the signal, the more the coil will resist passing the current. It is this characteristic that the coil is known for. When an AC signal is passed through the coil, the resistance becomes greater as the frequency increases. The result is that low frequencies and DC can pass easily. The highest frequencies are not passed as easily, hence the "blocking effect" of a coil for high frequencies. The way that a coil is used in a circuit changes the application. In one mode, the coil will reduce high frequencies. In another, it will short low frequencies to ground but let high frequencies pass. In the third mode, a coil can be used with a capacitor. The capacitor and coil will have a resonant frequency depending on the values of the coil and capacitor. In this mode, the coil and capacitor network will either pass or "block" only that frequency. This technique is used to either eliminate certain frequencies or pass only one frequency. Radio tuners are an example of this tuned circuit. The science and application of coils is extensive and whole books have been written about coils alone. WikiAnswers is perhaps not the best place to get detailed information but there are many resources on the internet that offer tutorials and calculators.

Answer 4

capacitor oppose a changein voltage across them by supply current as they discharged or charged.

the flow of current through a capacitor in thus directly proportional to the rate of change of voltage across it.

this is given by the relation

i=c*de/dt

where de/dt is the instantaneous change in voltage.

Answer 5

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Answer 6

Yes a capacitor can block DC and pass AC.