Hi guys,
Am a beginner..Finding it difficult to visualize things..Ma Question s..In capacitor,the current leads voltage by 90 deg..Right??So what does it mean??Like before the instant u apply a voltage u get a current??

 

hi,
You could also say,,, the voltage on the cap lags the current by 90deg.
Remember the Vcap rises exponentially,

Look at this link, it may help.

http://scipp.ucsc.edu/~johnson/phys6c/lecture25.pdf

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Right. So there is a current, which appears to be flowing through the capacitor, at the instant power is applied, but there is no voltage across the capacitor until it charges. (one plate with a large number of electrons, the other with a large absence of electrons.) In other words, it acts as a direct short at power up. Once the plates are charged, the apparent current stops flowing, and the voltage appears across its terminals, and it act like an open.

It will probably help to think of Voltage as Potential, and Current, as flow. If you think of potential energy (voltage,) verses kinetic energy (current,) then you should get some clarity.

There is a brick on the roof. It is not moving. It has a potential to do work. That potential is the distance above ground where the brick is sitting. Since the brick is not moving, it has no Kinetic energy. Similar to the potential difference which exists across the two wires in an outlet in your home. To continue, if nothing is plugged into the outlet, no current is flowing.

If the brick is falling the potential is being turned into kinetic energy.

Current appears to be flowing through the cap, until the plates are charged, at which point a potential difference appears across its terminals.

I hope that helps to clear it up.

Shoot, I'm slow. Eric appears to have posted while I was generating this silly opus.

 

Sorry about that.!

Hi BeeBop,
Check the wording of this section, is there a typo.

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Current is the only quantity here and voltage is just a potencial diference. Look on the time axis.
Although both the current and the voltage oscillate sinusoidally in an AC circuit they will not necessarily rise and fall simultaneously with each other in each circuit element or the circuit as a whole. The current and voltage will oscillate with the same frequency but they will (in general) be out of phase with each other. The exception being when the circuit is in resonace (phase angle of the circuit is 0) or if there is only resistors in the circuit.
In a pure inductor, the current lags the voltage by 90 degrees so if you look at the two wave forms (voltage and current) you will see the voltage at maximum at the same time the current is zero. As you introduce resistance into the circuit, the current lags by less.
In a capacitive circuit the current is said to lead the voltage although the reality is slightly different because a current happening before a voltage is applied is crazy. The truth is a negative current lagging by 90 degrees but the current leading idea works because the waves continue repeating.
Both reactances offer opositions to changes in voltage or current that is why always one lags the other.
It is useful to think of the phase angle as the angle by which the peak voltage leads or lags the current. If the phase angle is positive the voltage leads the current (inductive) by offering oposition to current changes. The oposite happens with a pure capacitive circuit. In a pure inductive or capacitive circuit the power dissipative is 0 but that never happens in the real world with all the variants.

I hope you get it. At least this is the way I understand it.

 

hi JD,
Makes sense to me.

I must admit I'm worried where BeeBop's falling brick is going to land.

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Hi Eric,

where should it fall? I guess it will fall on its versus.

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It must be a very high roof, I'm still waiting for the bang.!

BTW Hans, the 6P principle was General Pattens favourite quote.

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No, the brick will fall on the heads of those who think a MOSFET can only be used as a switch.

 

Thank u guys..Got it

 

The best way I know to explain the operation of a capacitor is to think in terms of its relationship to frequency. If you have a capacitor connected to a source through a switch, when the switch is closed, you have a sudden in-rush of currently. This translates to a very hight frequency (Xc = 1/(2πfC)). Because of the inverse relationship between reactance and frequency, this makes the reactance very small, which in turn causes the current to be at it's peak. Of course, as soon as the switch closes, the voltage begins to build from 0V across the plates of the capacitor. So while the voltage starts at 0V, the current is max and begins to decrease as the voltage increases. Then, when the voltage reaches it's max, the current is at 0 deg. From this, you can see the 90 deg phase relationship between voltage and current. Since current crosses 0 deg 90 deg before the voltage, it leads by 90 deg.

 

A capacitor is a bucket. A constant current source is a water hose. If you put the hose in the bucket, it fills up at a constant rate. If you put a current source into a capacitor, the voltage rises at a constant rate.

A capacitor is a bucket. A voltage source is a siphon from another bucket a little bit higher. At the beginning of filling the bottom bucket, the siphon is very strong due to the difference in water height. As the bottom bucket fills, the difference in water level height between the two buckets gets smaller, and the siphon strength gets weaker. The bottom bucket starts to fill more slowly the higher the level gets.

A capacitor is a bucket.

 

So, the brick falls in the bucket?

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Thats the reason why "there's a hole in the bucket dear Liza.!"

Thanks AllVol, I've always wondered why.

EDIT: I think we should have a Poll, is a Capacitor a Brick or a Bucket.?

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