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# Practical meaning of positive phase difference.

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#### AMS012

##### Full Member level 4
Hi,

In case of negative phase difference, it is understood that the output lags the input by the phase difference. But what is meant by positive phase difference (Ex. High pass filter)? Is it that output leads the input by that amount? If so, how can the output be there before the input??

Thank you.

It seems paradoxical, doesn't it? A capacitor in series causes current leading effect with sine waves.

The quick explanation is that the charged capacitor starts to discharge into the falling sine wave.

Current leading does not occur when the waveform is square.

I have a Youtube video which is an animated portrayal of current lead/lag effect in capacitors and inductors. It shows current bundles moving through wires. It shows the capacitor charging and discharging. It shows flux fields building and collapsing. It shows emf changing.

goldsmith

### goldsmith

Points: 2
Hi,

In case of negative phase difference, it is understood that the output lags the input by the phase difference. But what is meant by positive phase difference (Ex. High pass filter)? Is it that output leads the input by that amount? If so, how can the output be there before the input??
Thank you.

OK- lets take a C-R highpass as an example connected to a pure sinusoidal source.
At first, you have to consider the steady-state only, which means that all transients following the switch-on step have disappeared (By the way: Directly after switch-on at t=0 the current through the capacitor does NOT appear in advance to the exciting voltage).
At second, forget to think that the current through the capacitor is a kind of "loading" current.
It is a so-called "displacement current". And as such it reaches its maximum when the SLOPE of the voltage has it`s maximum - and this happens for crossing the 0-line (under steady-state conditions a constant voltage across the capacitor causes no displacement current). This effect causes the capacitor current to have a phase advance with respect to the corresponding voltage.

goldsmith

Points: 2