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I could expect sombody to ask " How capacitor trasmits AC?" but not this one. Anyway as was is said above the two capacitor plates are insulated from each other by a dielectric so net electric charges cannot cross the plates.
in the time varying case the dispalcement current is established between the plates of capacitor.
in the presence of the constant or DC current the displacement curent cannot be established between the plates.
the capacitor behaves like an open in the DC case since the current cannot pass throuhg the capacitor.
I could expect sombody to ask " How capacitor trasmits AC?" but not this one. Anyway as was is said above the two capacitor plates are insulated from each other by a dielectric so net electric charges cannot cross the plates.
For circuits with a constant (DC) voltage source and consisting of only resistors and capacitors, the voltage across the capacitor cannot exceed the voltage of the source. Thus, equilibrium is reached where the voltage across the capacitor is constant and the current through the capacitor is zero. For this reason, it is commonly said that capacitors block DC.
The current through a capacitor due to an AC source reverses direction periodically. That is, the alternating current alternately charges the plates: first in one direction and then the other. With the exception of the instant that the current changes direction, the capacitor current is non-zero at all times during a cycle. For this reason, it is commonly said that capacitors "pass" AC.
It may explain in terms of reactance also
For a capacitor Reactance is 1 / (2 x Pi x f x c)
Pi is numerical constant
F is frequency
C is Capacitance
For DC f is 0 so Reactance is infinite but in case of AC Reactance is low.
how capacitor blocks dc ? to get the answer of this question you have to know how the capacitor actually operates. initially when no potential has been apllied to the plates there is no field in the dielectric material and molecules are randomly oriented. when we apply a potential to the two plates of the capacitor, the dielectric material gets polarized, means the randomly arranged molecules get aligned in the direction of the field. theoritically it takes infinite time to align all the molecules in the direction of the field but practically nearly after five time constants the dielectric material gets polarized completely. so the capacitor gets fully charged. during the period of charging actually there is no current flow through the dielectric material. there is flow of current in the external circuitry but not in the dielectric material. this current is otherwise none as displacement current. so now let me answer your question. so when you apply a dc across the plates of the capacitor, for the initial five time constants there is displacement current. but after that period as the capacitor is fully charged there is no more polarization effect happening in the dielectric material. so there is no more current. and that is how capacitor blocks dc. now if you increase the dc potential above the dielectric strength of the material, then the dielectric material will get punctured and there will be heavy current flow across the plates through the dielectric material.
As pilu.sandeep was saying (ideal) capacitor only conducts displacement current but not conduction current.
If you apply a pure AC current to an ideal capacitor what happens is: inside the capacitor there is displacemnt current outside it there is conduction and they shall be equal as there is no energy escapes.
The displacement current is a current that can't be sensed expect in AC case as it is not a real flow of charges but it is a reorintation of either molecules vibration direction (or rotation axis) or the surronding electron cloud (all of them are affected in a back and force way not in translatory way-DC case-).
Also you can sink (in real due to the past stated facts)of the capacitor as a device that resists abrubt changes of voltage accros it , if a fast varying (AC) voltage is applied the voltage across it shall be tends to zero (short circuit) so it conducts AC while slow varying voltage the capacitor can follow them and so the capacitor impedance increases ;extreme case is the DC.
You might also want to take a look at this topic which is presently active
The topic of this thread is on the transient response of a capacitor, but your question has been answered as well. You might get a different perspective by reading the discussion there.
since the impedance of the capacitor is inversely proportional to frequency at zero frequency there is infinite impedance offered by the capacitor so it amounts to open circuit....so no current flows.......
Whereever we have insulating material and conducting material we get capacitance.{I remember that when I study introductry course in electromagnetics we were calculating the capacitance of a conductor configuration to infinity C∞}
The displacement currnt is the origin of the propagation of all electromagnetic radiation in insulating material including light propagation in air.
It is the addition done by Maxwell to the experiments done at his time to formulate his electromagnetic theory.
To investiget that the capacitance is every where and that it is only the problem of the frequncy => In low frequency circuts the design is easy ,
=> In microwave design all the design can collapse due the stray capacitance that is every where.
I hope that it becomes more clearer not more ambigous.
To sridhara,
capacitor doesnt allow DC but stores Electric fields.. It blocks DC , because the conduction in capacitor is due to DISPLACEMENT CURRENT (hope u know what it means)... By displacement current means , the opposite charges which deposited on the plate , changes when AC is applied..so the energy is stored for the amount of charge applied... But in case of DC , there ll be no charge polarity change and hence for DC , capacitor appears to be an open circuite(seperated by insulator)
A capacitor basically is a pair of metallic plates with a dielectric material in between. When a constant voltage appears across its terminals a movement of electric charges from one plate to another is produced. This movement only takes place for a short period of time. Once the electric charges from the source plate are depleted, then no more charges can move and the capacitor is said to be fully charged. In this condition no current can flow trough their terminals. The DC current is block.
the charging and decharging of a capacitor is depend on speed of changing polarization of voltage.
in dc because of not changing in voltage sign the capacitor will not decharge and It acts as a dc blocker
Added after 31 seconds:
the charging and decharging of a capacitor is depend on speed of changing polarization of voltage.
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