Sir, can you please make me understand, why a phase change occur while an ac runs through an inductor or capacitor.

 

That is a phase shift (a phase change is a chemistry term, meaning a change in physical state like ice melting) you are trying to learn about. Our Ebook has material that talks about these phase shifts. Start reading here - http://www.allaboutcircuits.com/vol_2/chpt_3/index.html

 

If you put too much AC into an inductor or a capacitor it will indeed exhibit a phase change as all the magic smoke floats out.

That is a change of phase from solid to gas referred to as "sublimation."

 

In a pure resitance the voltage and current are in phase.

If you apply a voltage to a capacitor through a resistor then max ccurrent flows to start with while the voltage is almost zero. This means the current is leading the voltage. But once the capacitor is charged the voltage is max and the current is zero.
With an inductor the opposite occurrs, the voltage leads the current.

 

Phase angle appears when voltage and current are not working in the same direction.