Capacitors block dc?then how it allows ac and how it works in ac circuit?pls explain?

Question

Capacitors block dc?then how it allows ac and how it works in ac circuit?pls explain?

Answer 1

As far as the flow of current is concerned, think of a length of pipe in a hydrauic system with a flexible membrane across it somewhere within that system.

Quite clearly a constant flow of current (d.c.) cannot take place, but at the instant pressure is applied the membrane will be distended.  This is similar to a capacitor charging.

However, if the direction of flow, from the system's pump, is continuously reversed, the membrane will first be distended in one direction and then the other, repetitively for as long as the alternating pressure from the pump is maintained.  

This cyclic change in pressure is present throughout the system (a.c.)

I have elaborated on this a bit, with diagrams, here http://www.davidbridgen.com/caps.htm

Answer 2

capacitors are used as filters.as we know in D.C,the frequency is zero,hence its reactance is 1/(2*pi*f*c){pi=3.14}so its reactance becomes infinity.hence becomes open circuit.but in A.C the frequency is not zero hence it behaves as short circuit..

Answer 3

WOW! not one person has yet mentioned electric fields.

There is a relationship between electric and magnetic fields.

There are magnetic effects associated with the changing electric fields between the two sides of the capacitor that are identical with those which would arise from the passage of actual current of this amount.  It can be interpreted as a current, but there is no physical motion of charged particles in the space between.

Answer 4

Imagine a night club with 2 entrance doors one on the left and one on the right separated by 20ft of pavement (sidewalk) 100 people are waiting outside to enter the club but do not know which door will be opened. They are evenly distributed between the 2 doors on the pavement (sidewalk) this is the equilibrium or uncharged state.

The doors represent capacitor plates the inside of the club represents the Insulation (dielectric) between the plates, the people represent electrons evenly distributed in the circuit, no current flow.

When a light is switched on at left hand door the people surge to the door and stop outside waiting for it to open. This is similar to the initial charging of a capacitor in a DC circuit. The door is closed and the people are blocked. Similarly the capacitor blocks the electrons in a  DC circuit

  

Imagine for fun the club decides to alternate switching on the lights at the left and right doors. The people will run from one door to the other back and forth, back and forth They do not pass through the left/right doors but if a third door was placed at a midpoint on the sidewalk and the people had to pass through this third door, when moving from left door to right door it would appear to that third door as an Alternating  flow

This is how to understand the capacitor in an AC circuit the electrons do not pass through the dielectric but move from one plate to the other through the circuit and any component in that circuit.    

Answer 5

Capacitors will not operate in DC. Capacitors as well as inductors carry something called impedance. impedance is resistance on the AC level. impedance is frequency dependant. Let us call w=2*pi*f where pi=3.14 and f is the frequency(given by 1/T where T is the period of a sine wave). The formula for calculating impedance of a capacitor is 1/(jwc) where j is the imaginary term square root(-1) and c is the capacitance. As you can see in DC the frequency is 0Hz. where 1/0=infinite. In engineering terms, that means that the capacitor acts like an open circuit in DC, not allowing any current to pass. However in high frequency applications, where f is a really large number, the capacitor acts like a short circuit.

Here is another FYI...

The impedance of an inductor can be found by the equation jwL where L is the inductance. In DC where w = 0 the inductor acts like a short circuit, allowing all current to pass just like a wire.(In ideal circumstances). In high frequency applications, the inductor acts like an open circuit, not allowing any current to pass through.

Answer 6

You may treat the capacitor as a resistor when it works in AC circuit.

If the frequency is unchanged, the bigger the capacitance the less resistance (reactance) it has. As the frequence increasing the same capacitor shall become less reactance. Other factors is phase angel when a capacitor works in AC,it shifts 90 degree advancing.

Answer 7

Capacitor are mad of two conductors that are not in direct contact but can influence one another.

In DC the current reach the first conductor and charge it, by influence the other conductor will be charged by opposite sign,so the current can flow in the rest of the circuit,till the saturation,or reaching equilibrium between the charge of the 2 plates.but since that happens too fast, we can say that the capacitor block the direct current.

In AC, there is never a constant potentiel in the first plat, so there is never a potentiel that the second plate can reach 2 ensure equilibrium, so the current will flow in the rest of the circuit.The only effect of the capacitor will be in slowing the potential in the rest of the circuit.