Self-Inductance and Inductors

SELF-INDUCTANCE AND INDUCTORS

Whenever a time varying current flows through any conductor it creates a "back-emf" that either opposes or sustains the current flow (Faraday's Law and Lenz's Law).
The "back-emf" is produced because the changing current in any small section of the conductor creates a magnetic field at all points in space around this small section. Some of the magnetic field will intersect the other parts of the conductor. Thus the changing magnetic field produced by one part of the conductor induces an additional current in the other parts of the conductor that add or subtract from the "original current" in the wire.
If I is increasing then the induced current produced substracts from the "original current" in that the measured current is smaller than it would be in no self-induced magnetic field was procuced in the conductor.
This process is called self-inductance since the changing circuit creates a "back-emf" in itself .
A circuit component that produces a large "back-emf'" are called inductors. The prototypical example being a solenoid.

The ideal inductors you see in a circuit diagram have no ohmic resistance. Any real inductor will have some resistance and can be modeled by a resistor and inductor in series with each other.

The definition of inductance is not physically intuitive:

L	= Inductance	SI: H = Henry = (V ^.s)/A = V / C
	= emf produced by the inductor	SI: Volt = V
dI/dt	= rate at which the current is changing in the inductor L	SI: A/s = Amps / sec

The inductance L is a measure of the size of the "back- emf" that given inductor will induce on itself.
The inductance L depends upon the physical structure of the circuit element and not upon the current or voltage across the inductor - L is a constant.
Only for the simplest of inductors (see the self-inductance of a solenoid) can one calculate the inductance using the above definition. For most inductors we experimentally measure their inductance L in same way that we determine the resistance R of a resistor by measuring its value.

We usually assume that we know the value of the inductance L and turn the definition around so we can find the voltage drop or gain across an inductor.

When there is a voltage drop across a resistor or inductor, the voltage is a negative number. However one often talks about voltage drop as though it were a positive quantity because the term "voltage drop" could be taken as a double negative. The voltage is negative and drop is negative.