it states that the voltage across an inductor is V = L(di/dt), why do we not use V = IR? Or what is the derivation of V = L(di/dt) or the conceptual explanation?
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Because Ohm’s law is not universal. It is a special case, that works for some electrical devices, and doesn’t work for others.
V = L(di/dt) is derived from experiment – you can take an inductor, run current through it, measure it, and see that it matches V = L(di/dt).
Ohm’s law is kinda like Aristotle’s law of motion: F=mv. It works, if there is a friction (moreover – a particular kind of friction). But if a friction is 0, or of a wrong kind – it breaks.
There are other devices that do not follow Ohm’s law. In semiconductors, the current is limited not by friction, but by an amount of free electrons – so they can have current saturation – when you increase voltage, but the current stays the same. Superconductors and vacuum have no friction at all – so they can have current without voltage.
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