I haven't closely followed this whole discussion, so
forgive me if I miss some previous contextual
point or am redundant.
Here's pretty much what you ought to know about
the concepts of practically understanding and
using inductance:
* Inductance measures the magnetic flux produced per
amount of current present in a circuit element.
* Knowing the current in and inductance of
and geometry of an inductor tells you the shape and
strength of the magnetic field around that inductor,
and that'd be what's relevant for something like an
electromagnet.
* Knowing the current and inductance of a
magnetically isolated inductor will tell you the energy
stored in the magnetic field of that inductor. This is
applicable mostly to single isolated 'coils' / 'inductors'
whose primary purpose is to have inductance,
and isn't generally the way you'd use / understand a
transformer overall since the main point of a transformer
isn't the inductance of its windings taken INDIVIDUALLY,
but is the COUPLING properties of signals or power
between its windings.
* Knowing the inductance of a magnetically isolated
inductor is also relevant if you're using that inductor
as a 'choke' or in a R-L or L-C filter or resonator circuit.
* Inductors that are designed to be highly coupled via
their magnetic fields to other inductors or conductive
objects in their environment are generally analyzed by
more attributes than just their inductance since the
significant magnetic field coupling and the uses that
is being put to is generally the most relevant thing
there. This would cover things like transformers,
metal detectors, et. al. When two inductors are
magnetically coupled they're said to have a
MUTUAL INDUCTANCE which is a true value of inductance
relating the field induced in the OTHER inductor by a
current in the FIRST. The mutual inductance between two
objects is a recriprocal thing, and depends geometrically
on the relative positions and windings of BOTH inductors;
it becomes not sufficient to talk about the INDIVIDUAL
inductances of coupled inductors since the presence of
a circuit on the coupled inductors effects the perceived
inductance of any single one of them.
Hence in idealized transformers you start to look at
things like "turns ratios" and "degree of coupling"
and "mutual inductance" as being mostly relevant rather
than the individual coil inductances ignoring coupling
effects.
* For isolated inductors in idealized cases you just
ignore the "self inductance" type of considerations
of how the magnetic field of the inductor itself
actually relates to the component's own coils etc.
Saying "the inductance of this inductor is L" is
enough to use that component in a simple circuit.
Using circuit equations relating to things like
time constants, resonance frequencies, Quality Factor Q,
complex Impedance, et. al. is directly possible knowing
JUST the (L) value of inductance. Similarly in
idealized cases you assume that there is NO relevant
coupling of the magnetic field of that inductor to
any other part of your circuit or conductive object.
Having a value of (L) and being able to say that it's
an isolated inductor is enough for all circuit analysis.
This FAILS to be true in 'extreme cases' of high
frequencies when you'll find that the value of Inductance
actually varies with frequency, and actually what one
naively assumes is basically a simple coil/inductor
does not behave like a PURE inductor but actually behaves
like a complicated little circuit containing resistance,
capacitance, and inductance all together. This is only
relevant for high frequencies usually above 50,000 Hz,
or for very poorly constructed inductors. For DC to several
kilohertz frequency analyses with well constructed
inductors, just use the inductance (L) in the equations
and it'll tell you all you need to know.
* So with respect to your concept of 'secondary' EMFs
or actually EMFs at all, that'd be mostly
an analysis you'd use to understand some kind of
magnetically coupled inductive circuit where some
given or assumed magnetic field acts as an 'external'
stimulus on some inductors or circuit coils/paths/loops
and generates an EMF. If you had a permanent magnet
and a loop of wire moving in its field acting as a generator,
or you wanted to design a transformer, etc. that's the
kind of analysis you'd do (coupled / induced EMFs).
For two-terminal simple circuit analysis, though,
you's treat each resistor (R), capacitor (C), inductor (L)
as a two-terminal device COMPLETELY specified by
its resistance, capacitance, inductance (respectively)
and you'd wholly IGNORE field-coupling considerations
due to coupled fields between circuit elements or the
influence of or generation of fields external to the
circuit, since that's beyond isolated circuit analysis.
You could of course use circuit analysis to analyze the
response of a circuit to a given 'external' EMF or 'external'
field causing an EMF in an inductor, though, if you were
building something like a generator driven circuit or
magnetic field meter or whatever.
* To the extent that you want to understand the
circuit analysis relevant ways that inductors,
resistors, capacitors behaves in a circuit, study things
like Ohm's law, the thevenin theorem, concepts of
complex impedance, phase angle, phasors, reactance,
parallel circuits, series circuits, resonant circuits, etc.
Basically there are very simple equations that tell you
how DC or single frequency sine wave AC voltage
sources behave when you have R, C, L circuits, and
how specific values of voltage and current become present
across each 2-terminal element (R, C, L) of the circuit.
In AC analysis you'll learn that inductive circuits have
voltage waveforms leading the current waveform in phase
(we're talking about single frequency sine waves here),
and capacitive circuits have voltage waveforms lagging
the current waveform. For purely resistive AC circuits
the voltage and current are in phase. Inductive reactances
are assigned to positive Y-axis reactance values in
the complex plane, capacitive reactances are assigned
negative Y-axis values in the complex plane, and
resistors get positive X-axis values in the complex plane.
So resistors have positive real number
resistance values associated, and capacitve reactances
are negative imaginary numbers, and inductive reactances
are positive imaginary values. Then you can do things
like use algebraic geometry to calculate phase angles,
vector magnitudes, parallel and series combinations.
You'll see that capacitive and inductive reactances in
series cancel out (because one is positive and the other
is negative), and that resonance can occur if capacitive
reactance is equal to inductive reactance, etc. etc. etc.
* Generally you only use EMF and magnetic field
based analysis for understanding motors, transformers,
sensors, generators, and electromagnetic theory in
general. Study magnetostatics, maxwell's equations,
electrostatics, lotentz force, ampere's law, lenz's law,
biot-savart law, etc. etc. to discover the ways you calculate
electric and magnetic fields due to charges and currents,
how fields add linearly in 2D/3Dvector space, how you'd
calculate inductance or capacitance of a geometrical
object, vector potential of the static magnetic field,
curl of the magnetic field, etc. etc. It's really more
about electromagnetic physics than relevant to circuit
analysis in that domain.
Mr_Bojingles said:
Ah yeah I got it mixed up. I was trying to understand how inductance is the property of opposing any change in current.
What I was reading is that when a changing magnetic field induces a voltage in a circuit that induced voltage is in the opposite direction of the voltage that caused the magnetic field.
For example I have a basic circuit. I apply 100 volts AC to it. This creates a magnetic field and since the AC current is fluctuating the magnetic field is also fluctuating. This changing magnetic field then induces a secondary EMF in the circuit but this secondary EMF is in opposition to the initial 100 volt EMF.
Have I got the concept wrong?
