MOTION MOUNTAIN

liquid electricity, invisible fields and maximum speed 59
F I G U R E 26 A Gauss rifle, made with a few steel balls and four magnets attached to a ruler with scotch
tape (© Simon Quellen Field).
Challenge 46 s
Challenge 47 e
Ref. 30
in mines were triggered when such a spark ignited a combustible gas mixture.
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Take an envelope, wet it and seal it. After letting it dry for a day or more, open it in the
dark. At the place where the two sides of paper are being separated from each other, the
envelope glows with a blue colour. Why? Is it possible to speed up the test using a hair
dryer?
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A charge in an electric field feels a force. In other words, electric field produce a potentialenergy
for charges. Since energy is conserved, electric potential energy can be transformedintokineticenergyorinthermalenergy.Whatdothesepossibilitiesallowdoing?
Whatdotheyprevent from doing?
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Electromagnetism is full of surprises and offers many effects that can be reproduced
at home. The internet is full of descriptions of how to construct Tesla coils to produce
sparks, coil guns or rail guns to shoot objects, electrostatic machines to make your hair
stand on end and much more. If you like experiments, just search for these terms. Some
people earn their living by showing high voltage effects on stage, such as long discharges
from their fingers or hair. A well-knownexampleisRobertKrampf,alsocalled‘Mr.Electricity’,
at thehappyscientist.com.Donotemulate theseperformers;it israrelytoldthat
several of them have suffered dangerous accidents while doing so.
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The moving discharges seen in so many displays, called plasma globes, are produced in
a glass bowl filled with helium, neon or another inert gas at low pressure, typically 0.1 to
10 kPa, an applied voltage of 5 to10 kV and usually a frequency of 30 to40 kHz. At these
conditions, the ion temperature of the discharges is room temperature, so that there is no
danger; the electron temperature, which cannot be felt, is around20000 K. Approaching
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net