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What causes impedance?
(lemmy.world)
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this post was submitted on 30 Aug 2026
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Electric current moving through a wire creates a magnetic field. Creating that magnetic field is work and takes energy. If it has to do a lot of work, that energy all gets stored in the magnetic field instead of in the load. Then if the electric current stops, the magnetic field collapses and dumps its energy back into the wire. That's impedance. It behaves like the electricity has been delayed and slowed down but it cannot be explained and understood as electricity alone like resistance or capacitance can, it is an electromagnetic phenomenon and can only roughly and imperfectly fit into an electricity-only model. The devil's in the details, and reactance is the devil's name.
Exactly. Physically, inductance and capacitance affect how quickly a circuit responds to a change in voltage.
A capacitor lets through current at first but pushes back as it fills up, with its energy stored in an electrical field.
An inductor doesn't like to suddenly change current, so it acts as if it has inertia where it won't let through current immediately upon a voltage being applied to it, and it won't immediately stop current flowing through it even if the voltage is taken off, because the energy being fed into the inductor gets stored as a magnetic field.
In a DC circuit, you can just map the changes to current in response to voltage changes, over time.
But in an AC circuit where the voltage doesn't stay still, the inductance and capacitance end up acting to shift the timing of the sinusoidal wave where the current and the voltage aren't timed in sync with each other.
So mathematically, because sinusoidal waves work well with phase angles and the math of imaginary numbers, it's easier to just think of the "resistance" as being a complex number with both a real and an imaginary component, and applying that math of complex numbers to the circuit behavior. And then we need new words to separate the real component (resistance), the imaginary component (reactance) and then both put together (impedance).
It's a mathematical model that very elegantly fits the physical reality of the time delay of how capacitance and inductance cause current to respond to changes in voltage. But it takes a step to explain why imaginary numbers are necessary to explain physical phenomena.