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 Inductors: What Good Are They?
The Properties that make a Conductor an Inductor are found in the following: 
If you take a Magnet and move it relative to a Conductor, a current is made to flow in that conductor. The magnitude of that Current is dependent on many things, including the Magnetic Flux density cutting the wire; the relative Speed of the Magnet; the number of turns the conductor makes, etc.  It can be said that the Moving Flux lines "cutting" the conductor's path induces an Electro Motive Force, an "E.M.F." --------
Conversely, if an electrical Current is passed through a Conductor, a Magnetic Field (Flux Lines) forms around the Conductor. 

With a sufficient number of turns of wire one has an Electro Magnet.

If one were to place two Conductors side by side; one passing a steady Electric Current; the other wire is unaffected. Although there are Magnetic Flux lines "cutting" the second Conductor, No E.M.F. is generated since there is No Relative Motion.

However, if the Electric Current is made to vary in Magnitude and/or Polarity, then there would be an Induction of an E.M.F into the second Conductor, i.e., this variation/change in Magnetic Flux has the same Effect as Relative Motion between the Magnetic source and the Conductor.

Thus far, we have covered the properties sufficient to create a Generator, a Motor and a Transformer.

The Generator and the Motor work by virtue of Polarity Reversal of strong Magnetic Fields; and will not be covered here.

   Step Function
   Counter E.M.F. Retards Current Flow
When a Voltage (step function) is applied to an Inductor, a current is made to flow through its conductors creating an expanding Magnetic Field (Flux).
This expanding Field induces a current (Counter E.M.F.) possessing an opposite polarity from that of the applied voltage. The amount of this counter E.M.F. is directly related to the derivative of the applied current, i.e., the faster the rise/fall the greater the current. The effect of this is to impede the rate of expansion of the Magnetic field.

Next, the Magnetic field stops expanding and becomes steady-state. At this point the Inductor is now only a Resistive load to the applied Voltage Source. 

Finally, the applied voltage is removed--made to equal Zero (Vapplied = 0 volts); the steady-state Magnetic Field now starts to collapse, which induces an E.M.F into the inductor thus causing a Current to flow (of the same polarity as that of the original applied voltage source). As in the applied case there is a Counter E.M.F. generated by the Current generated from the collapsing Magnetic Field, again impeding the collapse of said Magnetic Field.

The transfer of energy to the magnetic field of an inductor represents work performed by the source of the voltage. Power is required for doing the work, and since power is equal to Current multiplied by Voltage, there must be a voltage drop in the circuit while energy is being stored in the field. This voltage drop, exclusive of any voltage caused by the resistance (IR drop) in the circuit, is the result of an opposing voltage induced in the circuit while the field is building up to its final value. 
Once the field becomes constant, the induced voltage or back-voltage disappears, because no further energy is being stored. The induced voltage opposes the voltage of the source and tends to prevent the current from rising rapidly when the circuit is closed. The amplitude of the induced voltage is proportional to the rate at which the current changes (and consequently the rate at which the magnetic field changes) and to a constant associated with the circuit itself: the inductance (or self-inductance) of the circuit.
The effect that an Inductance has on Impeding current flow (due to the derivative of the applied current & the Inductance--measured in Henrys) is analogous to the Effect of Resistance on impeding current flow in a D.C. Circuit. However, this is a Reactive Impedance (Z); in this case Inductive Reactance (XL) measured in Ohms (Z =  XL).
 From the above, one can imagine if the Rate at which the applied voltage were fast enough, even a short piece of wire could exhibit relatively high impedances.    --And they Do! 
See Bypassing & Decoupling


Doubly Tuned LC Network
                      L C Filters
Low Pass Filter
High Pass Filter
Band Pass Filter
Band Reject Filter
(A.K.A. Notch Filter)
L C Tuned Circuit
Showing Xl, Xr, Xc
Ferrite Core Inductor



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