Lightning Experiment (Analysis)

Well, as it turns out, my idea of what I exactly I was trying to measure was completely wrong. Looks like I should have been doing something more like this:

This is probably one of the most confusing concepts in electromagnetics to understand. But here it goes:

During a lightning strike, a positive current flows from the ground up to the clouds. The current looks something like this:

If you refer to the picture of the lightning, I’ve drawn some magnetic field lines that have been induced by the current flow. THE STRENGTH OF THESE MAGNETIC FIELDS IS PROPORTIONAL TO THE CURRENT IN THE LIGHTNING. Unfortunately, I do not have equipment to measure the strength of magnetic fields directly.

I must rely on Maxwell’s law that says that THE CURLY ELECTRIC FIELD IS PROPORTIONAL TO THE DERIVATIVE OF THE MAGNETIC FIELD.

The curly electric field can be measured using the method shown in the first picture. Although, instead of using a DVM, it would be much better to use an oscilloscope since the voltage waveform will look like the derivative of current. You will observe a positive voltage as the lightning current ramps up to its maximum value, then a negative voltage as the lightning current begins to fall.

But let’s forget about my inadequate equipment and focus on what exactly causes the voltage that will be measured and analyzed. Look at how the DVM probe wires are arranged in a circle that is planar to the lightning strike. This allows the magnetic field lines to flow through the hoop. The voltage created along the hoop is proportional to the area of the hoop times the derivative of the magnetic field.

This can be said another way -> the voltage created along the hoop is proportional to the change in magnetic flux through the wire. Think of the hoop as a net that is catching the changing magnetic field. The bigger the net, the more of the magnetic field, and consequently, the more energy that gets absorbed by the field change.

If I had an oscilloscope, I WOULD BE ABLE TO FIND THE CURRENT OF THE LIGHTNING BY INTEGRATING THE VOLTAGE MEASURED ON THE HOOP and multiplying that value by some constant, determined by the distance of the strike, area of the hoop, and relationship between magnetic fields and current. IN ORDER TO DETERMINE THE VOLTAGE, I NEED TO KNOW THE HEIGHT OF THE CLOUDS AND THE STRENGTH OF THE STATIC ELECTRIC FIELD FROM THE GROUND TO SOME POINT IN THE AIR, similar to the bell experiment. In fact, if I were to measure the voltage between bell 1 and bell 2, that would tell me the electric field strength from the ground to the clouds. Using that, I could use the height of the clouds and determine the voltage of the clouds.

With my present setup (using the DVM as shown in the picture above), I will not be able to measure anything useful. However, it may be possible to add peripherals to the loop, such as diodes or amplifiers that will assist in getting a measurable waveform. In fact, that just gave me an idea… More to come on this later…

~ by daveinthewest on July 25, 2009.