pairofstrings
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When a capacitor is fully charged does it block AC or DC?
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hi pairofstrings! 
tell us what you think, and then we'll comment
tell us what you think, and then we'll comment
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What do you mean by "fully charged"?
pairofstrings
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By 'fully charged' I think it means that, more electrical energy cannot be accommodated beyond it's maximum point.
i think it should block DC, and allow AC if the circuit has a path to discharge the capacitor.
i think it should block DC, and allow AC if the circuit has a path to discharge the capacitor.
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pairofstrings said:
that's right!
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pairofstrings said:
But I want you to say what this "maximum point" is.
pairofstrings
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The maximum point is the point beyond which capacitor cannot be charged further.
I don't know why it happens that -
Capacitors can never get fully charged and they can never be fully discharged which is an immense point of interest.
Does anybody have any explanation for this phenomenon?
I don't know why it happens that -
Capacitors can never get fully charged and they can never be fully discharged which is an immense point of interest.
Does anybody have any explanation for this phenomenon?
DragonPetter
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pairofstrings said:
I don't know what you mean by fully charged or fully discharged. Can you explain?
Do you mean a "fully discharged" capacitor will never reach 0 volts DC across its terminals? If that is the question, it is just a matter of the RC time constant and that the solution to the differential equation governing this charge/discharge is exponential and so only its limit at t = infinity is 0. If you can have 0 resistance, the capacitor will discharge/charge instantaneously. You can simply apply a larger voltage across the capacitor and it will "discharge" to the level you want it to at some point in time. Of course this will be quantitized by the actual electrons and there are too many other non-ideal physics factors for this to be a real concern in most applications.
If you are talking about practical factors, a capacitor charges energy in the form of CV^2/2 where its capacitance is a function of the dielectric material used in the manufacturing, and there are many different capacitor technologies to store this energy. A capacitor is fully charged when it cannot hold any more energy without being damaged and it is fully discharged if it is brought back to 0 volts DC across its terminals.You can also think of it as the capacitor loses its charge, its voltage is dropping and so the electric field applied on the electrons decreases, and there is less force pushing the remaining electrons out, and eventually this force will be so low that it cannot move an electron. The only time a capacitor will stop discharging is if there is equilibrium of the electric field forces between the electrons in the capacitor and whatever it was discharged into.
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pairofstrings said:
The Charge on a Capacitor depends upon the Voltage across it and its Capacity so there is no fundamental limit
Q = CV.
But, of course, there is a limit to the voltage that you can put across a capacitor, which is basically two sheets of metal, separated by a small distance, with an insulating 'dielectric' between. Too high a voltage means that it will spark across, discharge and die (the dielectric has been punctured / burned permanently). This would be the 'maximum point' that I was trying to get you to come up with. There is no other maximum, though.
If you charge up a capacitor through a resistor (there is always a series resistance, however small), current will flow until the voltage across the capacitor is the same as the source. This is an exponential process and never 'really' gets there but it 'gets there' near as dammit after a period of a few 'time constants' (the product of R and C)
If you keep changing the direction of the applied voltage at a significantly faster rate than 1/(the time constant) then current (AC) will happily flow through the capacitor because it never manages to charge up before the direction reverses.
So we say it 'blocks DC' because current will stop flowing eventually but it let's AC through because some charge can keep flowing in alternate directions. The higher the frequency, the more current can flow.
The explanation why a capacitor never fully charges or discharges is that the current flowing into or out of it will depend upon the volts dropped across the series resistor (there is always one) the nearer it gets to being fully charged, the lower the voltage across the resistor and the lower the charging current. Therefore, it never quite gets there.
This is an exponential process - just like emptying a water tank through a hole in the bottom (water flow depends on head of water inside and the head becomes vanishingly small near the bottom so the rate of emptying gets less and less). When an object cools down, the same exponential process happens - heat loss is proportional to temperature difference. The nearer to the surrounding temperature the slower the rate of cooling. Again, the temperature fully gets to that of the surroundings.
Note: Mathematical Models only, of course!