Xref: utzoo rec.boats:4142 sci.physics:13677 sci.electronics:12992 Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!uwm.edu!zaphod.mps.ohio-state.edu!usc!elroy.jpl.nasa.gov!turnkey!orchard.la.locus.com!snap.la.locus.com!snap.la.locus.com!richard From: richard@locus.com (Richard M. Mathews) Newsgroups: rec.boats,sci.physics,sci.electronics Subject: Re: St Elmo's Fire (was Re: Lightning) Keywords: Induction, space charge, capacitance, point action Message-ID: Date: 20 Jul 90 06:11:14 GMT References: <1990Jul18.111525.5749@ioe.lon.ac.uk> Organization: Locus Computing Corporation, Los Angeles, California Lines: 74 The following description of lightning and lightning rods is not correct. The main error is the claim that "the earth is a natural source of negative charge." This means the end result about lightning rods repelling lightning is also wrong. >>I know this is the commonly accepted idea behind lightning rods, but as I >>learned in my college physics class, lightning rods actually REPEL lightning, >>rather than ATTRACTING it. The physics behind it goes something like this: >> The lightning rod MUST be grounded (electrically connected to the earth), >> otherwise it's useless. The other important feature of a lightning rod is >> that it is very pointy, not merely the highest point on a structure. >> A few basic facts need to be introduced before getting to the actual >> explanation. >> - Lightning is a discharge of excess negative (-) charge toward a source >> of excess positive (+) charge. >> - Unlike charges (+ & -) attract; like charges (- & - , or + & +) repel. >> - The earth is a natural source of negative charge. >> Okay, with those things in mind, the lightning rod is therefore (by being >> connected to ground) a source of negative charge. Due to the shape of the >> tip of the lightning rod being pointy (more precisely, a VERY small radius >> curve), the excess negative charge is radiated out from the tip of the rod >> in all directions. (an explanation of that gets rather technical) >> Therefore, with this field of negative charge radiating >> out from the rod, a bolt of lightning (also negative charge) is REPELLED >> from the rod, NOT attracted. Here's the real scoop (source: The Feynman Lectures on Physics, Vol 2, chapter 9). Away from thunderclouds, the earth has a negative charge relative to "the sky." The potential difference is quite incredible: about 100 volts per meter. Why does this not electrocute you? Because your skin is so much better at conducting than air, your skin can be treated as a perfect conductor. Your head and your feet have the same potential as the earth. A foot from your nose, the air is at a potential of +200V. The "battery" which keeps this enormous potential charged is all of the thunderclouds in the world. Relative to the earth, most of the bottom of a thunderhead has a negative charge (the top of the cloud and a section in the middle of the bottom have positive charges). Over most of the world it is fair to say that "the earth is a natural source of negative charge." Near the cloud, however, the earth is positive relative to the cloud. Lightning is a discharge of the supercharged negative portion of the cloud to the positive ground. A sharp lightning rod will concentrate the positive charge of the earth and bring it up closer to the source of negative charge. The lightning rods do attract lightning. A lightning strike begins with a dark leader which moves in steps towards the ground. This leaves a trail of ionized, negatively charged air which conducts well. When the leader gets close to a sharp grounded point (perhaps a hundred meters), the electric field near that point gets very large. The air ionizes there, and a trail of positive charge moves up to meet the lightning leader (well, negative charges progressively farther and farther from the lightning rod move down into the rod). When the trail coming up from the rod and the longer trail coming down from the cloud connect, the circuit is completed and the bright stroke of lightning moves up to the cloud. The current is about 10000 amps at its peak, and it carries down about 20 coulombs of negative charge. All of the negative charge from all of the lightning strikes across the world spreads out across the planet and produce the 100 volts per meter potential difference observed under clear skies. Since some regions of the world have frequent morning thunderstorms and other regions have evening thunderstorms, the potential varies through the day. On the average it stays around 90 V/m from 0h to 12h GMT with a minimum at 4h GMT. It rises steadily to about 120 V/m at around 19h GMT and then drops steadily back to 90 V/m at 0h GMT the next day. Note that the potential varies over the whole world at the same time -- the peak is at a specific Greenwich time, not local time. Richard M. Mathews Locus Computing Corporation richard@locus.com lcc!richard@ucla-se.ucla.edu