# AC capacitor circuit

Discussion in 'General Electronics Chat' started by uzair, Feb 7, 2008.

1. ### uzair Thread Starter Active Member

Dec 26, 2007
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0
In an AC circuit having a capacitor in it, the voltage lags by 90 degrees from current.How can we explain this phenomenon physically?
I mean what is happening with the flowing charges in that circuit, why the current is zero when votage is max?

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Let's say you had a flat tire on your car.

The air pressure in the tire is zero (relative to "atmospheric ground" at your barometric pressure, of course)

So, you connect an air pressure hose to fill up the tire. At the very beginning of filling the tire, the air flows into the tire very rapidly, but the pressure in the tire is still nearly zero.

As the tire begins to fill with air, you impatiently check the pressure in the tire. What? It's not full YET? I checked the pressure 20 seconds ago, and it had 10 PSI in it after filling it for 20 seconds. Now it only has 13 PSI in it!

As the pressure in the tire builds, the air flowing into it becomes slower and slower, because the pressure differential between the air supply and the pressure in the tire is becoming less and less. Therefore, it takes longer and longer to get an increase in pressure in the tire.

In this example, the air pressure is a direct analogy to voltage. The air flowing through the hose, the valve stem and finally into the tire is analogous to current. In the beginning, the air flow was at maximum because the pressure in the tire was nearly zero. As the pressure built up, the current flow became less and less.

Does that make sense?

3. ### uzair Thread Starter Active Member

Dec 26, 2007
110
0
Thanks SgtWookie it really helped.
Can you give a similar one for AC inductor circuit as well?

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
You are having an argument with your wife. Your wife, as a woman, has infallible intuition, and her intuition makes tremendous headway. As a man, your logic is impeccable, and your logic does not move. Slowly, your logic sways her intuition. BUT the effort makes you emotional. By the time her intuition catches up with where your logic was, you are frustrated and passionate. A that point, she is calm and her mood is even. Your frustration eventually inspires her back to intuition and passion. The effort tires you and you become cold and logical again.

Men are metaphors for EMF. Women are metaphors for current. Domestic arguments are metaphors for inductors.

5. ### uzair Thread Starter Active Member

Dec 26, 2007
110
0
Thanks for the analogies guys they were damn good!
Actually you guys saved me from a lot of cramming.I learnt all the concepts and was getting ready to cram.Thanks a lot.