Lightning Question

[Bill Roosa]

Agree Walt

You don't have any control.

However, figuring it the other way
dV/dt = L * dI/dt ---> dI/dt = (1/L) dV/dt
Given a lightning strike current (dI/dt) if you increase the inductance you would have to also increase dV/dt. So you get a high voltage change at the point where the inductance increased. That is what I'm looking for, a high change in voltage through the inductor keeping the high voltages out of the salon and up on the mast and rigging where my low impedance path to the surface takes care of it.
So the high dV/dt at the mast base keeps the bottom of the mast at "lightning potential" while the mast support in the salon has a much lower potential. Basically the coil's magnetic field chokes off the current causing a charge pile up at the mast base and a high voltage there.

Thoughts?

 

[Ross]

Re: Agree Walt

Bill , the only problem with your theory is finding an inductor rated for 100 kilovolts.

 

[T37 Phil]

In my thinking of lightning I'm making the following simplifications to the situation:
a) The lightning current in air is acting like a long straight wire with no insulation. In/on the boat all bets are off

Ok.

b) My boat is attached to what amounts to an infinite capacitor

Not grounded, right? Why infinite C? The capacitance depends on the width and material of the dielectric.

c) The induced static electrical charge is a very thin (<<1 ft) sheet at the water surface.

Ok, there is a concentration of charge that does not run deep.

The E field for a lightning bolt looks like horizontal lines of equal potential.

It would be equipotential planes, but not for the bolt, for the clouds that spawned the bolt. The field for the bolt/wire has not been defined by any poster yet.

The H field looks like sets of concentric circles centered on the lightning bolt. Which would be in this case horizontal circles since the lightning bolt is vertical.

Ok.

Since my boat is in the initial E-field it has a masthead that is at some "more positive" potential, the waterline is at the "most negative" potential and the keel is at some "more positive than the water surface but less than the masthead" potential. No current flows though because the whole boat and water surrounding it are in the same back to back E field.

When the current begins to flow it is primarily in the vertical direction. I want to encourage that and try and discourage horizontal currents (side flashes).

Anybody disagree with me yet?

Yes, I'm going to disagree. You are using the water surface as a field generating charge surface, but then you talk about the potential within this 'black box' charge source. That's not correct. The field near the boat (top and bottom) is more complicated than you are describing, I believe.

By the way - you are using the wrong inductor equation.

 

[T37 Phil]

Yup.. a little confusing as there are a few threads.. but I was reference to post 40 and 46 in this thread.

Ok, I reread #40 now too. I think you need to replace potential everywhere with charge, and then reevaluate. It's not clear to me why the boat would act capacitively and store up charge that would be dissipated later to ground. If it did, anything that could dissipate would have experienced currents during the charge build up phase first, and that would look like a 'direct hit', I imagine.

 

[walt]

Ok, now I dont know what your talking about..

 

[T37 Phil]

It all starts with charge, all of it.

In thinking about the plates of a capacitor, it is the circuits externally attached to the plate leads that force the movement of charge on or off the plates. Fields are merely a mathematically description of what charges exist in the neighborhood of the problem space. Potential is a property of fields (not just electrical fields). So the use of potential in #40 is misleading, in addition to the observation that I really think he's talking about charge.

So, if we're talking about charge and capacitance, then we're talking about two opposing surfaces (or some other contorted geometry requiring explanation) and a dielectric medium that separates them with some known width. Charge is forced on the plates by a current flowing one direction resulting in a final charged voltage and accompanying actual charge stored. When the influences that forced the charged onto the plates is gone, the capacitor discharges, that is, current flows in the opposite direction to/from the plates and the voltage returns to it's previous level. There are many, many graphs of such things on web pages describing capacitors.

All that's missing for the picture is what are the plates where the charge is stored, what is the nature of the dielectric separating them, and by what path(s) are they charged/discharged during the event? I'm not saying it isn't there, but I don't see the full picture yet.