Why inductive or capacitive load lag or lead current by 90 degree?

From first principles and trying to make it really simple. In an inductive circuit, the difference in potential causes a current to flow. As the current flows it induces a magnetic field. If this magnetic field in interacting with other magnetic fields (Itself or another) the other magnetic field suppresses the buildup of the magnetic field that this current is trying to establish. This inhibits the current rise as the voltage increases. In a DC circuit this happens at switch on and to a lesser extent when the load varies. If the voltage (load) is stable, the current will be stable and thus the rising and falling magnetic fields are not a major problem. In an AC circuit at 50Hz, this rise and fall is happening 100 times a second. This causes the current to lag behind the voltage that is producing it.

In inductive circuits there will always be an element of resistance so pure inductance is impossible to achieve without super conductors. In capacitors it is possible to get closer to a 90 degree leading current as the resistance, capacitance ratio is very large as the resistance is very small.

It is the basic property of these components like which can be seen from their equations v =l(di/dt) and v=idt/c as we apply voltage to inductor it opposes change in current because change in current induce EMF in coil/inductor so voltage appears first and then current and because its resistive component is negligible that is why the phase difference is 90 degree and when we apply voltage to capacitor then current flows in it first and its terminal voltage increases as the current flows (accumulation of charges due to current flow on plates) so here also resistance is negligible and phase difference is 90 degree.

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