MOTION MOUNTAIN

56 1 electricity and fields
Challenge 37 s
Challenge 38 s
Challenge 39 s
Challenge 40 s
Challenge 41 e
transport power across longer distances. Why?
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How can you distinguish a magnet from an non-magnetizedmetalbar of thesamesize
andmaterial, usingnoexternalmeans?
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In thebasement of a housethereare threeswitches thatcontrol three light bulbs in the
firstfloor.Youareinthebasementandareallowedtogotothefirstflooronlyonce.How
doyoufindoutwhichswitchcontrolswhichbulb?
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Howdoyouwireupalightbulbtothemainsandthreeswitchessothatthelightcanbe
switched on at any of the switches and off at any other switch? And for four switches?
Nobodywill takeaphysicistseriouslywhoisabletowriteMaxwell’sequationsbutcannotsolve
this little problem.
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The first appliances built to generate electric currents were large rubbing machines.Then,
in 1799 Alessandro Volta (b. 1745 Como, d. 1827 Como) invented a new device to generate
electricity and called it a pile; today its basic element is called a (voltaic) cell, a primary
cell* or, less correctly, a battery. (Correctly speaking, a battery is a collection of cells,
as the one found in a car.) Voltaic cells are based on chemical processes; they provide
much more current and are smaller and easier to handle than electrostatic machines.The
invention of the battery changed the investigation of electricity so profoundly that Volta
became world famous. At last, a simple and reliable source of electricity was available
for use in experiments; unlike rubbing machines, cells and piles are compact, work in all
weather conditions and make no noise.
An apple or a potato or a lemon with a piece of copper and one of zinc inserted is one
of the simplest possible voltaic cells. It provides a voltage of about1 V and can be used
to run digital clocks or to produce clicks in headphones. Volta was also the discoverer
of the charge ‘law’q=CU for capacitors (C being the capacity, andUthe voltage) and
the inventor of the high sensitivity capacitor electroscope. A modest man, nevertheless,
the unit of electrical potential, or ‘tension’, as Volta used to call it, was deduced from his
name. A ‘battery’ is a large number of voltaic cells; the term was taken from an earlier,almostpurelymilitaryuse.**Abatteryinamobilephoneisjustanelaboratedreplacement
foranumberofapplesorpotatoes.
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Voltaiccellsexistinallbiologicalcells.Forhalobacteria,theinternal voltaic cells are even
essential to survival. Living in saltwater, internal voltaic cells help them to avoid death
due to osmosis.
*Asecondarycellisarechargeable cell.
** A pile made of setsof a zinc plate, a sheet of blotting paper soaked with salt water and a copper coin is
easilyconstructedathomeandtestedwith acalculator oradigital watch.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net