I am looking for help with a question about volt amps vs watts and I think I understand that in an AC circuit with reactive components the waveforms of the voltage and current are not in phase. My question is , in short, why? What is happening at the electron level that causes some of the current to be returned to the supplier? What causes the voltage and current to be out of phase and why does it then cause current to be returned?

 

Just look at the equations that govern the relationship between the voltage and the current in an inductor and a capacitor. In the first case the voltage is the derivative of the current and in the other the current is the derivative of the voltage. The reason is that these are "reactive" components, meaning that they store energy instead of just converting it to heat, like a resistor does.

Note that it's not a mater of current being returned to the supplier -- the current leaving the supply is always equal to the current returning to the supply (this is known as Kirchhoff's Current Law). I think the notion you are looking for is that the circuit returns energy to the source.

 

Since equations don't readily show what's actually happening, let's do a little thought experiment and look at the capacitor current versus AC voltage applied in the time domain:

As the sinewave voltage starts to increase, current flows in the capacitor charging it, with the current determined by the rate-of-change (slope) of the sinewave voltage. The higher the slope, the higher the capacitor charging current.
When the sinewave reaches its peak voltage the capacitor will have charged to its maximum value and the current will stop since the voltage slope is zero (unlike a resistor which would be drawing the maximum current at the peak voltage).
Then as the voltage starts to drop the capacitor will start to discharge, returning current back to the AC source. This current will now reach it's peak value as the sinewave crosses zero since that is where the voltage is changing most rapidly (maximum slope).

Thus the current of a capacitor leads the voltage by 90° for an AC sinewave voltage across it.

A similar though experiment can be done for an inductor, which I'll leave as an exercise for the reader.

 

Grab a textbook from the library.

 

I SUGGEST THAT YOU CAN READ THE TEXBOOKS, CHAPTER 3 AND CHAPTER 4(ALTERNATING CURRENT) REGARDING AC REACTANCE AND IMPEDANCE -INDUCTIVE AND CAPACITIVE. THIS WILL GIVE YOU MORE DETAIL EXPLANATION .
YOU CAN READ MANY TIMES AS YOU WANT UNTILL YOU UNDERSTAND IT.

THANKS..

 

Hello,

@JUNELER , Please do NOT use all capital characters.
It will be seen as shouting, wich is not nice.

Bertus

 

thanks for the replies, especially crutschow's. that's exactly what i was looking for.

 

A simple waw to think about it is that the inductor or capacitor is storing and releasing energy on every cycle- it takes time to charge a capacitor or inductor. The current will lead or lag according to when it can release that stored energy back into the circuit. With a resitive impedance no energy is being stored, hence voltage and current are in phase.