Re: [Phys-L] lightning

[John Denker <jsd@av8n.com>]

On 06/12/2013 04:30 PM, Bill Nettles wrote:

> ..... this "current of positive charges" racing upward has
> me puzzled. What are these positive charges, oxygen ions? This
> explanation sounds fishy/wrong to me, but I've never studied
> lightning or the literature about it. 

Let's start with this, borrowed from Feynman:  Suppose there are
a bunch of cars, all in one lane, all headed northbound, stopped 
at a red light.  When the light changes, the first car starts 
moving.  A little while later, the second car starts moving, and 
so forth.  There is a "wave of starting" that propagates southward 
through the group of cars.

Note the distinction between the /wave/ and the /medium/.  The
wave is moving southbound, even though every bit of the medium
is either stopped or moving northbound.

======

So it is with a certain /part/ of the lightning stroke.  Imagine
a coax cable, several kilometers long, open-circuited at the bottom
end.  Via some mechanism at the top end, it gets charged up.  There
is a big voltage observed at the terminals at the bottom end.

Now short-circuit the bottom end.  A wave will propagate upward
along the transmission line.  The wave speed will be a good fraction
of the speed of light, determined by the inductance per unit length
and capacitance per unit length of the cable.  There is also some
resistance per unit length, but I don't have time at the moment to
go into that.

Now let's refine the model.  Strip away the outer conductor, so that
all we have is the inner conductor.  It's still a transmission line,
with some self-inductance per unit length and some self-capacitance
per unit length.  There are some sketchy logarithmic divergences, but
let's not worry about that too much.

Also, this conductor is not made of copper, but rather plasma.  It
contains positive ions, negative ions, and free electrons.

Now we can answer the question that was asked:  The free electrons
will have far greater mobility than the ions.  If free electrons are
present in any appreciable numbers, they will dominate the current.
They will move downward (in the most common type of cloud-to-ground
strike).  However, that is the motion of the /medium/.  The motion
of the /wave/ is upwards.

For more info, try something like this:
  http://www.google.com/search?q=lightning+capacitance+%22per+unit+length%22

Note that Uman has been working in the field for decades and knows
what he's talking about.

More later.  Gotta run now.