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4 Oct 2009 21:14:11 IST

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This one is conceptual: When we charge a capacitor using D.C. Source then current will flow in the

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This one is conceptual:When we charge a capacitor using D.C. Source then current will flow in the circuit till the capacitor is fully charged but how is it possible when we know that a capacitor completely blocks D.C. (capacitive reactance is inversely proportional to the frequency of curent flowing)

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djdylan2000. bits goa eee

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5 Oct 2009 00:08:47 IST

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When we say a capacitor completely blocks DC, we mean it does so in the steady state, not in the transition phase.

edison

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5 Oct 2009 10:30:32 IST

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The charging of capacitor is almost instantaneous with time constant = RC

Thus after this transient state is overcome at steady state the impedance of capacitor becomes infinite for DC as capacitive reactance is inversely proportionla to frequency of applied voltage.

Charging a Capacitor

When a battery is connected to a series resistor and capacitor, the initial current is high as the battery transports charge from one plate of the capacitor to the other. The charging current asymptotically approaches zero as the capacitor becomes charged up to the battery voltage. Charging the capacitor stores energy in the electric field between the capacitor plates. The rate of charging is typically described in terms of a time constant RC.

Mukul Chakravarty

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5 Oct 2009 12:33:11 IST

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Thanks man.

Dharav Solanki

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5 Oct 2009 20:31:55 IST

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Yeah dude - Good question. Think of it! Capacitors block DC. But why do they block DC? They block DC because they have a constant voltage. Blocking DC means that there is no current through a circuit branch.

This is because, the capcitor has its own voltage. When you apply voltage to an uncharged capcitor, it gains charge and then aquires a voltage. [What voltage is given by the formula V = Q/C?] So, the total potential across a capcitor becomes zero after it is fully charged.

Put this question in another words and you'll realiza that it is not a capcitor that blocks DC but a capcitor charged to its full capcity that blocks DC. Otherwise, tell me one thing - how will you charge a capcitor if it blocks DC?

Mukul Chakravarty

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6 Oct 2009 15:52:50 IST

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Thanks Dharav. I didn't think of that

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