MOTION MOUNTAIN

16 1 electricity and fields
F I G U R E 2 Objects surrounded by fields: amber (c. 1 cm), lodestone (c. 1 cm) and mobile phone
(c. 10 cm) (© Wikimedia, Philips).
Challenge 3 s
comb
rubbed
on wool
water
pipe
F I G U R E 3 How to amaze kids, especially in dry weather (photo © Robert Fritzius).
withthosetypesofmotionwhicharegeneratedwithoutanycontactbetweenthebodies
involved. Thiscanhappeninthreeways.
Fields: amber, lodestone and mobile phones
You can always surprise children with the effect shown in Figure 3: a comb rubbed on
wooldeviatesrunningtapwater.Thesameeffectcanbeproducedwithanair-filledrubberballoon
rubbedonwool.Everybodycandeviate waterstreamswithoutanycontact.
TheGreekshadalreadyobservedthiseffectalongtimeage.Infact,thestoryofelectricity
starts with trees. Trees have a special relation to electricity. When a tree is cut, a
viscousresinappears.Withtimeitsolidifiesand,aftermillions ofyears,itformsamber.
Whenamberisrubbedwithacatfur,itacquirestheabilitytoattractsmallobjects,such
as saw dust or pieces of paper. This was already known to Thales of Miletus, one of the
original seven sages,in thesixth century bce.The sameobservation can be made with
many other polymer combinations, for example with combs and hair, with soles of the
shoeoncarpets,andwithdustandacathoderay tube inside an oldtelevision.Another
interesting effect can be observed when a rubbed comb is put near a burning candle.
(Canyouimaginewhathappens?)
Another part of the story of electricity involves lodestone, an iron mineral found in
certain caves around the world, e.g. in a region (still) called Magnesia in the Greek
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net