Question on ac circuits?
Answer:
Unlike a resistor that opposes flow of current, an inductor opposes a CHANGE in the flow of current.
It's easiest to understand with straight AC and a sine wave, and it would sure be nice to be able to include graphics here!
The voltage dropped across the inductor is due to its reaction against the change in current through it. So the instantaneous voltage is zero whenever the instantaneous current is at a maximum, and the instantaneous voltage is at a maximum peak wherever the instantaneous current is at maximum *change* (where the sine wave has the steepest slope, crossing the zero line). The difference between the peak and zero crossing are 90 degrees apart, and this is what causes a voltage wave that is 90 degrees out of phase with the current wave.
Edit: Finally found a site with a straightforward graphic for you to see:
http://www.allaboutcircuits.com/vol_2/ch...
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It takes a quarter of a cycle to get the inductor up to speed. This is so quick that you can for all practicality consider this instantaneous.
Because of the impedance of the inductor.
That's the only way the math works out.
V/I=Z, where Z is the impedance of the inductor Z=sL in complex frequency. V being volts and I being current.
I gave you a start, you do the proof.
If you want a physical explanation, I would say that it takes time for the magnetic field (the energy storage component in an inductor) in the inductor to change. V=L * dI/dt.
Imagine I(t) = sin (t), then V = L*cos(t)
and cos(t) = sin(t-90)
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