10 Jun
2009
10 Jun
'09
7:32 p.m.
Dave McGuire wrote:
.. we tend to coast along with 'adequate' models for day to day efforts.
Having an appreciation for the underlying principles helps
when things don't work as expected, amongst other benefits.
--
OT: Too easily distracted from things I should be doing, here are
two ballpark approaches to determining the capacitance of a cloud:
- by cloud and earth as (perfect) plates:
Given a cloud with a base of 2 km diameter, situated 3km above the ground,
(p is permittivity of air), then:
C = pA/d = 8.9e-12 * 3.14*(2000/2)^2 / 3000
= 0.009 uF
- by energy content:
If a lightning strike delivers 30 KWh of energy at 200 million volts, then:
E = 1/2 CV^2
C = 2E / V^2
= 2 * 30,000 Jh/S * 3600 S/h / (2e8 J/C)^2
= 5.4e-9 C^2/J
= 0.005 uF
E & V parameters were just some numbers picked off web sites about lightning.
Interesting they came out with a rough equivalence.
Going by such web sites, there isn't really that much energy in a lightning
strike (it's just very high power as it is released so quickly.)
Search for
"dielectric absorption", or look up "permittivity" in
Wikipedia to
get an idea of just how complex capacitors are (!)
Yes, I'm aware of their complexity...From my childhood days I
thought they were simple, but in recent years I've studied them
pretty closely. I've been dabbling in metrology for a few years now
and have become amazed at the complexity behind the "simple" things
that are the underpinnings of all things electronic. It's
fascinating. Anyone interested in electronics would do well to
investigate this a bit.