also i think similar topic has been discussed here before (it had a cool java simulation) i hope u find that using forum search.The current through a capacitor due to an AC source reverses direction periodically. That is, the alternating current alternately charges the plates: first in one direction and then the other. With the exception of the instant that the current changes direction, the capacitor current is non-zero at all times during a cycle. For this reason, it is commonly said that capacitors "pass" AC. However, at no time do electrons actually cross between the plates, unless the dielectric breaks down. Such a situation would involve physical damage to the capacitor and likely to the circuit involved as well.
Since the voltage across a capacitor is proportional to the integral of the current, as shown above, with sine waves in AC or signal circuits this results in a phase difference of 90 degrees, the current leading the voltage phase angle. It can be shown that the AC voltage across the capacitor is in quadrature with the alternating current through the capacitor. That is, the voltage and current are 'out-of-phase' by a quarter cycle. The amplitude of the voltage depends on the amplitude of the current divided by the product of the frequency of the current with the capacitance, C. ----------wiki
(Q = VC).Hello,
Can anybody tell me why current leads voltage by 90 degree in capacitor and vltage leads current by current in inductor?
Also please suggest me some good books regarding this.
Thanks in advance,
Movva.
If you measure the voltage and current phase between a filter's input and output, then yes, the phase changes with frequency, due to the effect of the resistance in the circuit. But the voltage measured directly across the capacitor is still 90° out of phase with the current through the capacitor, independent of frequency (of course the voltage isn't). It's the phasor addition of that phase shift and the resistor phase shift that determines the filter output phase shift. For LP RC connection you posted, at high frequencies the capacitor reactance is much lower than the resistor value so the phase shift approaches 90 degrees. At low frequencies the capacitor reactance becomes very high and so the phase shift approaches zero degrees.If current lags voltage then why is vout in phase with vin at low frequencies for a pass filter. The current through the resistor is always in phase with the voltage across it, which is in phase with both Vin and Vout. As it's a series circuit, then the current through the capacitor is the same as that of the resistor, which means current through the capacitor is also in phase with Vout. This seems contradictory to these previous posts.
No. How do you get from the voltage across the resistor (i.e., Vin-Vout) being in phase with the current in it to a claim that it is now somehow always in phase with either Vin or Vout, let alone both?The current through the resistor is always in phase with the voltage across it, which is in phase with both Vin and Vout.
While this explains why voltage and current are 90 degrees out of phase in either a capacitor or an inductor, it falls short of explain why voltage leads current in one and the opposite in the other. In either one, when current is at a maximum (amplitude) the voltage is zero and, likewise, when current is zero voltage is at a maximum (amplitude).Non mathematically, it's because the current depends on the rate of change of voltage, not the actual voltage across the capacitor.
So with a sine wave, maximum current occurs when the voltage is passing through zero. Draw a graph and you will see the 90deg phase difference!
With a inductor, the voltage depends on the rate of change of current - due to self induction. Or to put it another way: the rate of change of current depends on the actual voltage across the inductor.
See?
The situation you describe only occurs at DC (frequency equals zero) in which case the capacitor looks like an open circuit, no current is flowing in the resistor, and Vout equals Vin.Hmm. Vin and Vout are in phase at low frequencies. There will come a time when Vin and Vout are both 0v at the same time. At this time, if current in capacitor is leading by 90 deg, then the current in capacitor is peaking. If it's a series circuit, current through resistor should be peaking too. But by definition both ends of the resistor are now at 0v (Vin = Vout = 0v). How can current through it be peaking, it would have a voltage drop across it?
Veni
by Duane Benson
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