Quote:
Originally Posted by Rudy
Hmm, interesting. I would surmise that during normal weather (not a thunder storm), the source resistance for this 200V/m electric field must be very high since the air molecules are mostly not ionized and the air therefore acts as a dielectric medium.
The presence of the human hunter must then have a significant effect on the electric field in his immediate surrounding, given his relatively low resistance, specially on a hot humid day. One can model the human resistance roughly as an outer resistance and an internal body resistance, where the skin resistance is of the order of 2,000Ω or less if sweaty, and the internal body resistance on the order of 500Ω. The resistance to ground (forgetting shoes for the moment) is then:
Rbody = Rskin(in) + Rinternal + Rskin(out)
Since we get from the external skin to the lower resistance internal organs and back to the external skin.
So Rbody ~4.5 KΩ maybe less if sweaty.
Assuming a 2 meter height for the human, We would effectively have 400 V from head to feet and Ohm's law would say that we'd have almost 90 mA of current flowing through us, a lethal amount.
Of course, we won't die because the source impedance behind that electric field is so high that it can't provide that kind of current. But, wouldn't our mere presence be sufficient to collapse that electric field in our vicinity?
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Hi Rudy,
Yes the air is considered one of the best insulators and a dielectric which basically makes the earth a capacitor with an opposite pole at the ionosphere.
When we consider the resistance of a person internally and externally, and even the resistance of his shoes, this resistance becomes a moot point because of the tiny amount of current that is flowing through the resistance of the air. The amount of current which normally leaks through the air is only around in the area he occupies is around 1 nA, which would quicly move from the ground through a person;s body before it met any noticable resistance that could develop a potential in his body. At this current level, the person (including most ordinary shoes he would be wearing) basically acts like a conductor, which raises the ground potential to his level. This is especially true on humid days. If we are talking about a very dry day where a person walks across a carpet to generate a sizable charge, then he could build up thousands of volts. A lot more than 1 nA will be leaking from him into the atmosphere in this condition. But without generating a charge by this method, only a tiny trickle of 1 nA will charge a person, that would require better insulating value than the air to keep from seeing him as a conductor and preferred path to the ground. Also consider that simply by being there, the person may double or triple the leakage current to 2-3nA in the surface area he occupies. However, in this same area, he has caused the voltage gradient to make an enormous anomaly where it dropped from 200v to 0v! ... Why it is better to measure anomalies in the voltage gradient.
Another way of looking at it is a person standing on the ground (or plant, or any other partially conductive object) will raise the ground potential from the ground up to the level of their body. In essence, a person will distort the ground potential by simply being there. By standing on the ground, we have created the effect of a small hill shape on the ground to raise the ground potential to a higher altitude. Thus, we are not standing in a 100-200 volt/meter gradient because our body caused the gradient to drop much lower, nearly to ground potential along the full height of our body. This may be easier to understand when you consider a vertical cliff maybe 100 meters tall. We will se ground potential at its base and at the top, as well as all along the face of this cliff. We can expect the vertical voltage gradient of 100-200 volts/meter to be distorted to a horizontal gradient when measuring it at the face of the cliff. This distortion will be expected to gradually return to the normal vertical gradient as we move away from the cliff.
You can see an illustration of this from the Dr. Feynman's book of his physics lectures Vol 2 chapter 9 -- online copy of this page here:
http://student.fizika.org/~jsisko/Kn...Atmosphere.pdf
Anything on the ground is basically a lightning rod/grounding rod unless it's resistance is somewhere close to the resistance of the air. And this is a fortunate state of affairs because it prevents people from inadvertently dying due to electric shock from accumulated atmospheric charge in non-storm conditions.
But as a final thought, When we consider the big influence of everything from trees, people, hills, wild animals, lakes and streams in influencing the atmospheric voltage gradient, it seems more likely that a more conductive area of ground could have a significant effect on the gradient.
Best wishes,
J_P