MOTION MOUNTAIN

Chapter 5
ELECTROMAGNETIC EFFECTS
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Looking carefully, the atmosphere is full of electrical effects. The most impressive,
ightning, is now reasonably well understood. However, it took decades and a
arge number of researchers to discover and put together all the parts of the puzzle.
Also below our feet there is something important going on: the hot magma below the
continental crust produces the magnetic field of the Earth. Strong magnetic fields can
be used for levitation. We first explore these topics and then give an overview about the
many effects that fields produce.
Is lightning a discharge? – Electricity in the atmosphere
Inside a thunderstorm cloud, especially inside tall cumulonimbus clouds,* charges are
separated by collision between the large ‘graupel’ ice crystals falling due to their weight
and the small ‘hail’ ice crystallites rising due to thermal upwinds. Since the collision takes
part in an electric field, charges are separated in a way similar to the mechanism in the
Kelvin generator. Discharge takes place when the electric field becomes too high, taking
a strange path influenced by ions created in the air by cosmic rays. (There are however,
at least ten other competing explanations for charge separation in clouds.) It seems that
cosmic rays are at least partly responsible for the zigzag shape of lightning. For a striking
example, see Figure 152.
A lightning flash typically transports 20 to30 C of charge, with a peak current of up
to20 kA. But lightning flashes have also strange properties. First, they appear at fields
around200 kV/m (at low altitude) instead of the2 MV/m of normal sparks. Second,
lightning emits radio pulses.Third, lightning emitsX-raysandgammarays.Russianresearchers,from1992onwards
explainedallthreeeffectsbyanewlydiscovered discharge
mechanism. At length scales of50 m and more, cosmic rays can trigger the appearance of
lightning; the relativistic energy of these rays allows for a discharge mechanism that does
not exist for low energy electrons. At relativistic energy, so-called runaway breakdown
* Clouds have Latin names. They were introduced in 1802 by the explorer Luke Howard (b. 1772 London,
d. 1864Tottenham), who found that all clouds could be seen as variations of three types, which he called
cirrus,cumulusand stratus. He called the combination of all three, the rain cloud,nimbus(from the Latin
‘big cloud’). Today’s internationally agreed system has been slightly adjusted and distinguishes clouds by
the height of their lower edge. The clouds starting above a height of6 km are the cirrus,the cirrocumulus
and the cirrostratus; those starting at heights of between 2 and4 km are the altocumulus, the altostratus
and the nimbostratus; clouds starting below a height of2 km are the stratocumulus, the stratus and the
cumulus.Therainorthundercloud,whichcrossesall heights,istodaycalled cumulonimbus.Forbeautiful
viewsofclouds,seethewww.goes.noaa.gov andwww.osei.noaa.gov websites.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net