Whenever a source of voltage either DC voltage or AC voltage is connected across a capacitor C, the electrons from the source will reach the plate and stop. They cannot jump across the gap between plates to continue its flow in the circuit. Therefore the electrons flowing in one direction i.
DC cannot pass through the capacitor. But the electrons from AC source seem to flow through C. Let us see what really happens! Consider a parallel plate capacitor whose plates are uncharged same amount of positive and negative charges. A DC source battery is connected across C as shown in Figure a. As soon as battery is connected, electrons start to flow from the negative terminal and are accumulated at the right plate, making it negative. Due to this negative potential, the electrons present in the nearby left plate are repelled and are moved towards positive terminal of the battery.
Skip to content. Last updated: 7th Nov ' Why does the capacitor blocks DC but not AC? So changes on one side of the barrier provoke changes on the other side, so that it appears that the charges cross the barrier, and that current effectively flows through the capacitor.
A charged capacitor is always DC charged, i. These charges are a storage for electrical energy , which is necessary in many circuits.
The maximum voltage is determined by the insulating barrier. Above a certain voltage it will breakdown and create a short circuit. That can happen under DC but also under AC. A simple way of thinking about it is that a series capacitor blocks DC, while a parallel capacitor helps maintain a steady voltage.
This is really two applications of the same behavior - a capacitor reacts to try to keep the voltage across itself constant. In the series case, it's quite happy to remove a steady voltage difference, but any abrupt change in one side will be passed through to the other to keep the voltage difference constant. In the parallel case, any abrupt change in voltage will be reacted to. This is not a very technical answer, but it's a graphical explanation that I find very funny and simple:.
The amount of charge that develops across the plates of a capacitor with a given voltage across its terminals is governed by the formula:. The voltage across the plates of a capacitor must also change in a continuous manner, so capacitors have the effect of "holding up" a voltage once they are charged to it, until that voltage can be discharged through a resistance.
A very common use for capacitors is therefore stabilize rail voltages and decouple rails from ground. The voltage rating is how much voltage you can apply across the plates before the electro-static forces break down the material properties of the dielectric material between the plates rendering it broken as a capacitor :. My answer to such questions is always "water".
Water flowing through pipes is a surprisingly accurate analogy for current flowing through wires. Current is how much water flows through a pipe.
Voltage difference becomes the difference in water pressure. The pipes are supposed to lie flat, so that gravity plays no role. In such an analogy, a battery is a water pump, and a capacitor is a rubber membrane which completely blocks the pipe.
DC is water flowing constantly in one direction through a pipe. AC is water flowing back and forth all the time. With this in mind, it should be obvious that a capacitor blocks DC: since the membrane can only stretch so far, water can't just keep on flowing in the same direction. There will be some flow while the membrane stretches i. It also becomes obvious that a capacitor won't block AC completely, but it does depend on the membrane properties. If the membrane is sufficiently stretchy high capacitance , it will pose no challenge to water flowing back and forth quickly.
If the membrane is really rather stiff e. This analogy has been so exceptionally useful to me that I really wonder why it isn't used more widely. By "blocking", we mean than it offers a high impedance to the signal we're talking about. First, though, we need to define a few terms to explain this. You know what resistance is, right? Resistance is the opposition to current flow that results in the burning of power, measured in watts.
It does not matter if the current is AC or DC, the power dissipated by a perfect resistor is the same amount for either. So resistance is one kind of "impedance" to current flow.
There are 2 others - "inductive reactance", and "capacitive reactance". Both are also measured in ohms, like resistance, but both are different in that, for one thing, they vary with frequency, and for another, they don't actually consume power like a resistance does.
So all together, there are 3 kinds of impedance - resistive, inductive, and capacitive. Where 2pi is approximately 6. Inductive reactance is the impedance of a component due to inductance; it is a kind of resistance, but does not actually burn power in watts like a resistor does, and since "f" for frequency needs to be supplied, the value of it varies with frequency for a given inductor.
Notice that as the frequency goes up, so does the impedance AC resistance in ohms. And notice that if the frequency equals zero, then so does the impedance - a frequency of zero means DC, so inductors have virtually no resistance to DC current flow. DC is a constant value i. Now lets connect the capacitor in DC and then AC and see what happens?
Keep in mind that a capacitor act as a short circuit at initial stage and a fully charged capacitor behave as an open circuit. Capacitors resist a changes in voltage while inductors resist a change in current and acts as a short circuit in DC. At initial stage when we connect a capacitor to the DC supply, there will a small current of flow will occur until the plates becomes saturated.
In other words, the positive terminal of DC supply source will suck the electrons from one terminal and push the electrons to the second terminal until the first plate becomes positively charge and the second one as negatively charged as shown in fig.
At this stage, the applied voltage equal to the voltage across capacitor and capacitor plates are saturated and there is no more flow of current. At this stage, capacitor behaves like an open circuit and if we increase the value of applied DC voltage, the capacitor may damage and explode. We know that there is no frequency i. It means, theoretically, a capacitor will provide infinite resistant to the flow of current according to its rating.
Hence no current flow will occur as current in capacitive circuits are:. If we put X C as infinity, the value of current would be zero.
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