# Capacitor & Inductor

Discussion in 'General Electronics Discussion' started by vick5821, Mar 13, 2012.

1. ### vick5821

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Jan 22, 2012
Why capacitor is open circuit under dc condition and inductor is short circuit under dc condition ?

So far I just memorise these facts until today my lecturer mentioned about it..

Can anyone help me ? Wan to understand more

2. ### jackorocko

1,284
1
Apr 4, 2010
"Under a dc condition" means a stable voltage.

An ideal capacitor has zero current flowing through it (acts like a open switch, open circuit) when the voltage across it is the same as the DC voltage source.

An ideal inductor is nothing more then a wire. Once charged, The inductor still acts like a wire ( like a closed switch, short circuit).

Last edited: Mar 13, 2012
3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
Consider an inductor to be a length of wire to DC, and a capacitor to be a length of wire with a break in it to DC.

Once things reach equilibrium, that's effectively what they are.

Both will exhibit other characteristics briefly as a DC voltage is applied because the increase from 0V to some voltage can be seen as (kind of) part of an AC signal.

4. ### vick5821

700
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Jan 22, 2012
so what are you trying to say ?

Thank you

5. ### vick5821

700
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Jan 22, 2012
I understand for the capacitor part..no Potential different = no current flow..but for the inductor part , any more details ?

6. ### jackorocko

1,284
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Apr 4, 2010
I don't think you understand. When a capacitor first charges the voltage across it's two leads is zero (short circuit). When a DC voltage is applied across the capacitor it starts to charge. The voltage across the two leads and will increase till it reaches the voltage applied to it. When the ideal capacitor is fully charged their is no current going through it, the capacitor is acting as a (open circuit)

If you think about how a capacitor and inductor are made it may make more sense. What is an inductor made out of? What is a capacitor made out of? When these two components are in a stable DC condition, they act exactly how you would expect a wire (inductor/closed switch) and two metal plates separated by an insulator (capacitor/open switch) to act.

7. ### Laplace

1,252
184
Apr 4, 2010
When you first apply voltage to a capacitor, the capacitor appears to be a short circuit. But as current flows the electrostatic field builds up intensity and then decreases to zero as the field intensity reaches maximum.

When you first apply voltage to an inductor, the inductor appears to be an open circuit. But as current starts to flow the electromagnetic field builds up intensity and then the current reaches its maximum as the field intensity reaches maximum.

In each case, energy is stored in the resultant electric field.

8. ### vick5821

700
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Jan 22, 2012
I understand what you meant..Initially when we apply DC voltage, there is potential diff between the source and in capacitor.so it is short and current flows..and when it is fully charged up and the voltage of capacitor is the same as source, no potential difference and hence no current flow..thus open circuit..Am I correct ?

Thank you