Current through coils
 

Different currents at each end of a coil are easily explained.

A coil has capacitance to its surroundings distributed along its
length. And current flows from it radially.

Every coil has length. Both L and C are distributed. Therefore the
coil behaves as a transmission line. There are standing waves. Current
and voltage both vary with length.

The longer the line the greater the variation.

The line has phase-shift, Zo and attenuation resulting from wire loss
resistance and radiation resistance.

The line can have a resonant length. It then becomes a short antenna.

The whole business can be mathematically modelled.
----
Reg, G4FGQ.

Current through coils
 

Reg Edwards wrote:
The whole business can be mathematically modelled.

The point is that it cannot successfully be modeled
with the lumped circuit model where the current is
constant everywhere in the circuit. What would
Kirchhoff have thought about a coil with 0.1 amp
at the bottom and 0.7 amps at the top? It certainly
doesn't mean that 0.6 amps is flowing sideways. All
it means is that the relative phase of the forward
current and reflected current changes through the
coil and therefore the distributed network model
needs to be used.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil,

Interesting.

I am quite familiar with Kirchoff's equation in regard to voltages
around a loop and his equation about currents at a node. Did I miss a
third equation regarding currents around a loop?

Hint: Kirchoff would not be the slightest bit bothered by this problem.

Reg, as usual, has it completely correct.

73,
Gene
W4SZ

Cecil Moore wrote:
Reg Edwards wrote:

The whole business can be mathematically modelled.


The point is that it cannot successfully be modeled
with the lumped circuit model where the current is
constant everywhere in the circuit. What would
Kirchhoff have thought about a coil with 0.1 amp
at the bottom and 0.7 amps at the top? It certainly
doesn't mean that 0.6 amps is flowing sideways. All
it means is that the relative phase of the forward
current and reflected current changes through the
coil and therefore the distributed network model
needs to be used.

Current through coils
 

Gene Fuller wrote:
I am quite familiar with Kirchoff's equation in regard to voltages
around a loop and his equation about currents at a node. Did I miss a
third equation regarding currents around a loop?

Nope, you missed an implication of Kirchhoff's current law.
Unequal currents into and out of a passive black box implies
the existence of a node inside the box.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg Edwards wrote:
The whole business can be mathematically modelled.

IF one uses the correct model. Here's an email I received today.

Walter Maxwell, W2DU wrote:
Hi Cecil, I just today found the (QRZ.com) discussion. I agree
with your position 100%. It's as simple as this:

If an inductance is in series with a line that has no reflections,
the current will be the same at both ends of the inductor.

If an inductance is in series with a line that has reflections,
the current will NOT be the same at both ends of the inductor.

Consequently, circuit analysis will not work when both forward
and reflected currents are present in a lumped circuit.

When reflections are present, a current node and a current loop
can appear at separate points on an inductor simultaneously.

Walt
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil,

Your response makes no sense at all. Unequal currents into and out of a
passive black box implies charge storage, which generally means capacitance.

You cannot have it any other way, with or without waves or reflections.
Conservation of charge is one of the most fundamental laws in nature.

Reg was correct.

73,
Gene
W4SZ



Cecil Moore wrote:
Gene Fuller wrote:

I am quite familiar with Kirchoff's equation in regard to voltages
around a loop and his equation about currents at a node. Did I miss a
third equation regarding currents around a loop?


Nope, you missed an implication of Kirchhoff's current law.
Unequal currents into and out of a passive black box implies
the existence of a node inside the box.

Current through coils
 

Gene Fuller wrote:
Your response makes no sense at all. Unequal currents into and out of a
passive black box implies charge storage, which generally means
capacitance.

Boundary condition: There's nothing but wire inside the black box.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Gene Fuller weote:
"Conservation of charge is one of the most fundamental laws of nature."

Unequal currents into and out of a passive black box are very simple to
produce. Suppose the black box contains a simple transformer with a
ratio other than one to one? The power can be constant but the voltage
and current must be different on input and output. R-F is a-c, not d-c.

Best regards, Richard Harrison, KB5WZI

Current through coils
 

Richard Harrison wrote:
Gene Fuller weote:
"Conservation of charge is one of the most fundamental laws of nature."

Unequal currents into and out of a passive black box are very simple to
produce. Suppose the black box contains a simple transformer with a
ratio other than one to one? The power can be constant but the voltage
and current must be different on input and output. R-F is a-c, not d-c.

Best regards, Richard Harrison, KB5WZI

Now explain how you'd do it with a box having only two terminals -- and
assuming the box is very small compared to a wavelength.

Roy Lew

Current through coils
 

Roy Lewallen, W7EL wrote:
"Now explain how you`d do it with a box having only two terminals--"

I`ll give the mathematician`s answer: "It`s of no interest. It`s already
been solved." Cecil said he would put a coil in the box. I agree.
Retardation between incident and reflected waves in each direction would
in most cases cause a current difference between the two ends of the
coil. Unlike the usual transmission line, the wire is coiled to get
reactance into a small space. The effect is the same in that phase shift
is distributed along the length of the wire. There is just more of it
and and the intervals between maxima and minima are short. Impedance and
therefore voltage along the vire is a function of site along the wire.
There will be a standing wave pattern throughout the coil.

Best regards, Richard Harrison, KB5WZI