Re: Ive got this problem with AC through capacitors (phase lag)

From: Bill Bowden (wrongaddress_at_att.net)
Date: 11/10/04 

Date: 10 Nov 2004 09:12:26 -0800

Steve Evans <smevans@jif-lemon.co.mars> wrote in message news:<irj2p0td9n3nt3keue3l824bqonaq7do5t@4ax.com>...
> Helo,
>
> If Ive got a resistor, a battery and an ammeter, and I stick the
> battery across the resitor, I get a certain number of milliamps
> current going through the resistor.
>
> If I increase the voltage thruogh the resitor, I get more current.
>
> If I decreas the voltage, the resistor passes less current.
>
> So far, so good.
>
> Now, if I try to do the same thing with a capactor instead of a
> resistor, I get an initial curent flow that tapers off over time as
> the cap charges up and winds up with the same voltage acsross it as
> the battery.
>
> Again, so far so good.
>
> Now, if I try to stick a AC voltage accross the capacitor; everything
> goes tits-up. For part of the cycle, yeah, the current increases with
> the applied voltage and for other parts of the cycle, it decreases
> along with deminishing voltage.
>
> Fine again. (well not really -see below)
>
> The problem is, at *other* parts of the cycle, the current has started
> to reverse direction whilst the voltage is still increasing! And the
> bit further on again, the same thing happens vice-versa. Whats going
> on here??

Yes, but at other parts of the cycle, the source voltage
is still higher than the capacitor voltage, so the cap will
continue to charge even though the source is falling.

The capacitor will stop charging and begin discharging
*exactly* when the source voltage falls to the same
potential as the capacitor.

All you have to do is think about the difference in voltage
at various points along the waveform to determine the direction
of current.

>
> And how can current *lead* voltage? YOu have to have a voltage to get
> a current to flow in the first place!!
>

You don't really need a voltage to have a current flowing.
Think about a tank circuit with a capacitor and inductor
exchanging energy at some frequency. The current in the
tank circuit will be maximum when the voltage across the circuit
is zero, and visa versa.

-Bill

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