Thursday Jan. 13, 2011
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First day of class. We started with some information about the course, a
project
that you will have the
option of doing, and mentioned some books
that might be used during the
class. A list of topics that
might be covered during the semester wasn't covered at the beginning of
the class but we came back to it briefly during the period. Note
the that examples of midterm
exams and final exams
from previous editions of the course are accessible via links on the
main class
webpage.
A quick overview of the Global Electric Circuit is a good way to
introduce much of what we will be covering this semester. The
following notes are a more neatly drawn version of what we did in class.
Point 1
Together, the earth's surface and the ionosophere resemble a spherical
capacitor The two electrodes are the ground and the
ionosphere. The ground is negatively charged during fair
weather. Positive space charge is found in the air between the
ground and the ionosphere (most of
the charge is near the ground) instead of on the second
conductor. The positive charge is attached to particulates in the
air (aerosols) and is relatively immobile (due to the large size and
large inertia of the particles). These are called "large ions."
Point 2 You would find a
downward pointing electric field of 100 to 300 Volts/meter at the
ground during normal fair weather conditions. We will assume that
the ground is a perfect conductor in many of the problems we do, so the
field will be perpendicular to the ground. Also we will often
just consider the ground to be flat because we will just be looking a
small portion of the globe and because the atmosphere is much thinner
than the radius of the earth. That's why we used Ez in the
picture instead of Er.
Points 3 & 4 Air is a
very
poor conductor but
does have a finite conductivity. A very weak current flows from
the ionosphere to the ground. Jz
in the figure is current density
( amperes/meter2 ).
Point
5 Estimate of Jz.
We can multiply this by the area of the earth's surface to
determine to total current flowing between the ionosphere and the
earth's surface.
Point 6 a bit of a
detour. Why don't you feel the 400 volt potential difference
between your head and toes when standing outside on a fair day?
Because air has a high resistance, a very weak current flowing
through air can produce a large potential difference. The
resistance of a human body is much lower. The person is
effectively a short circuit and there is little or no head-to-toe
potential difference.
Point 7 The potential
of the ionosphere ranges from 150 kV to 600 kV relative to the earth's
surface (see Table 15.1 in The Earth's
Electrical Environment i.e. the "yellow book") We'll use an
average value of 280,000 volts (instead of the 200,000 volts used in
class).
We can divide the surface-ionosphere potential difference by the
current flowing between the ionosphere and the surface to determine an
effective resistance of the atmosphere.
Point
8 The following equation shows the relationship between
surface charge density and electric field at the surface of a conductor
(we'll derive this expression soon in class, if I remember right it is
a simple application of Gauss' Law)
The following calculation shows
that it wouldn't take very long for the current flowing between the
ionosphere and the ground, I, to neutralize the charge on the earth's
surface, Q.
This doesn't happen. The
obvious question is what maintains the surface-ionsphere potential
difference? What keeps the spherical capacitor charged up?
Point 9 The original
answer was lightning. Most cloud-to-ground lightning carries
negative charge to the ground.
At some point it became clear that lightning alone wasn't enough.
The thinking then became thunderstorms in general. Point (b)
shows an upward current flowing from the top of the thunderstorm and
also from point discharge currents on the ground. These currents
aren't quite sufficient.
The current thinking is that thunderstorms and non-thundery but
electrified clouds are needed to produce sufficient charging current.
Points 1-8 in the figure at the tops of today's notes (yellow) form
what might be called "fair weather atmospheric electricity."
We'll spend a significant portion of the class (40% perhaps,
maybe somewhat less depending on the interests of the students in the
class) discussing this topic.
Point 10 The majority of
the class will be devoted to stormy weather electricity, i.e.
thunderstorms, lightning, and related topics. We'll look at how
thunderstorms become electrified (doesn't it seem surprising that
electrical charge is created and separated in the cold wet interior of
thunderstorms?). We'll spend quite a bit of time looking at the
sequence of events that make up negative cloud-to-ground
lightning. We'll also look at other times of lightning
(intracloud lightning, positive cloud-to-ground lightning, upward and
triggered lightning). Lightning currents can be measured directly
and indirectly (from the radiated fields). Some knowledge of
lightning currents characteristics is needed to be able to design
effective lightning protection equipment. And we'll cover
lightning protection and lightning safety.
I'm planning to doing some basic demonstrations and also bringing
in some examples of working instrumentation used in thunderstorm and
lightning research. The plasma globe that appeared at the end of
today's class seemed appropriate for the first day of class.
You'll find a clear and basic explanation how plasma globes work here.
