Lightning Revisited [RE-wrenches]

[Antony Tersol]

Lightning involves two separate phenomena:

Electrostatics: the negatively charged bottoms of clouds INDUCE a
positive charge on the surface of the earth below.  Lightning is an
electrical discharge between the clouds and oppositely charged ground or
between oppositely charged parts of clouds.

Electromagnetic inductance: currents produce magnetic fields.  Changes
in magnetic field induce other currents.  A lightning strike is a very
large current.  If that large a current was constantly flowing, the
constant magnetic field would not induce other currents.  Because the
strike is sudden and short, there is a large change in current,
therefore a large change in magnetic field.  This change results in a
large change in magnetic flux through any circuits that are nearby. 
This change in flux INDUCES a current through those nearby circuits.

Lightning nearby can inductively produce currents in circuits.

Some of the confusion arises because physicists use the term INDUCE in
both phenomena.  

See inline comments below.

Drake wrote:

At 11:09 AM 4/16/02 -0700, you wrote:
>>Drake -
>>
>>Inductance works not by raising the potential of each of the conductors
>>a set amount all along the conductor.  Inductance causes an
>>electromotive force AROUND a circuit which is proportional to the time
>>rate of change of the magnetic flux through the circuit.  The "magnetic
>>flux through the circuit" is the amount of magnetic field through the
>>area of the circuit.

>That is why I used the example of 2 wires in a conduit.  Both conductors 
>would be exposed to approximately the same flux.  The magnetic field would 
>cut through the conductors equally, as they would be in very close 
>proximity.  All along their length they would be getting equal exposure as 
>the magnetic pulse rose and fell.

The individual conductors are not exposed to the flux.  The circuit is
exposed to the flux.  It is the area between the conductors times the
strength of the magnetic field that IS the flux.
The importance of the wires being close is that the area is smaller and
therefore the flux is smaller.  If the magnetic field were uniform over
a wide area, by your argument wires could be separated by a distance,
yet still get "equal exposure".  That is false.  It is the area between
the wires times the magnetic field that constitutes the flux, and it is
changes in the flux that  create the EMF (Electromotive Force).

>Code, in general, requires wires of the same circuit to be run 
>together.   Both wires would rise in voltage at the same rate.  Voltage is 
>relative.  Ground potential is arbitrary.

Both wires do NOT rise in voltage at the same rate because of
electromagnetic inductance.  Those changes are static electripotential
effects caused by the buildup of electric charge.  Inductance comes from
changes in magnetic field, which results from the movement of charges
(the lightning strike nearby), and causes an EMF AROUND the circuit
(along the wires, not between the wires).  This INDUCES a current around
the circuit, so that the current is going one way in one wire, and the
opposite way in the other wire.  It does not cause a rise in voltage at
the same rate along the length of the wires.  These electromagnetic
inductive currents are the currents that caused the damage at the
Solarex facility referred to earlier by Bill Brooks. 


>>   That is why the problems with a large series
>>circuit in which the wire returning current from the extreme panel does
>>not return along the same path as the wires connecting the panels.

>That would definitely be a problem.

It can still be a problem, even with the wires in close proximity if
they are simply parallel, because there will still be some flux, though
significantly less than when they are separated by a large distance.  It
will be much less of a problem if the wires are spiraled: the area
between adjacent points where the wires cross can be considered a loop;
the flux of adjacent loops will cancel because the current around
adjacent loops is in opposite directions.  See diagram below.

Arrows indicate direction of current

    >             >             >
------------------------------------- conductor 1 >>>>
                                      direction of magnetic field into
paper
------------------------------------- conductor 2 <<<<
    <             <             <
    A             B             C

flux through each part of circuit = separation x segment length x
magnetic field

If each segment is L long, and separation is S, and magnetic field is B,
then
each flux is LSB, and for three segments flux is 3LSB


    >             <             >
----------   -----------   ---------- conductor 1 >>>>
           X             X              direction of magnetic field into
paper
----------   -----------   ---------- conductor 2 <<<<
    <             >             <
    A      ^      B      ^      C
        twist          twist
       in wires       in wires 

Now the flux in segments A and C is still LSB each, but the flux in
segment B is -LSB,
because the current around the miniloop of B is in the opposite
direction to what it was
in the first example.  Therefore the total flux through all three
segments is 2LSB - LSB or a total of LSB.  Adjacent miniloops will tend
to cancel, so the total flux through a twisted wire pair will tend to
zero, so that even with very strong magnetic field changes, the induced
current will also tend to zero.   

Antony Tersol

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