about capacitor.....

:lol: sorry for this stupid questions guys.......

first we know that a capacitor blocks dc and pass ac. Also a filter capacitor smooths the out ripples in a circuit.

but how could we measure a dc voltage in a capacitor how that happen?
how come a dc volatge is measure but as we know that a capacitor blocks dc.... pls. help me guys I want to be enlighthen.......................

jin29_neci said:

:lol: sorry for this stupid questions guys.......

first we know that a capacitor blocks dc and pass ac. Also a filter capacitor smooths the out ripples in a circuit.

but how could we measure a dc voltage in a capacitor how that happen?
how come a dc volatge is measure but as we know that a capacitor blocks dc.... pls. help me guys I want to be enlighthen.......................

Click to expand...

"Capacitance is the property of a circuit to store energy in the form of an electrostatic field"

Think of a capacitor as a kind of small rechargable battery!.

To understand the "block DC, pass AC" concept you might read up on capacitive reactance.

Reactance might be described as the resistance to the flow of current much like a resistor. For a capacitor the resistance to the flow of current is high at low frequencies and drops as the frequency increases. DC might be thought of as an infinitely low frequency so one would expect the resistance to the flow of current to be infinitely high.

Keep studying, you'll soon understand.

I think what jin29 is saying is that he accepts a capacitor has DC blocking properties but doesn't understand how it's possible to read DC across a capacitor if it's supposed to block it.

What you'll find is that yes, you can read DC across a capacitor because a charge will build up on one plate relative to the other. Both plates of a capacitor will always store an equal and opposite charge. The plate at the higher voltage will always carry the positive charge, and the plate at the lower voltage will always carry the negative charge.

The difference between a capacitor and a resistor is that charge will continuously flow through a resistor (which is what causes current flow) whereas charge does not directly flow through a capacitor - it simply sits on the capacitor plates, the gap between them preventing any direct flow of charge. The only time charge will flow from one plate to the other is during a discharge, which under DC conditions the capacitor isn't going to do. It's only when you start talking about AC conditions that you'll get a continous charge/discharge situation and therefore only under AC conditions will charge (and therefore current) flow.

This is why a capacitor "blocks" DC. You'll still measure a voltage across it because the capacitor plates will become charged as a result of the potential difference across them, but once charged the gap between the plates prevents any actual charge flow. If you were to connect a capacitor and a resistor in parallel and then connect a DC potential difference at each end, once the capacitor had fully charged the only current flowing would be through the resistor.

I hope this clears things up a bit, and that I haven't confused you further... I also hope none of my explanation is too inaccurate

Brian

jin29_neci said:

first we know that a capacitor blocks dc and pass ac. Also a filter capacitor smooths the out ripples in a circuit.

Click to expand...

You probably need to better understand the role of capacitors than these 2 statements. The first statement describes capacitors removing anti-change, while the second statement describes capacitors removing change. Both statements are correct, but the capacitor is used in different ways.