MOTION MOUNTAIN

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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? ∗∗ How can you distinguish a magnet from an non-magnetizedmetalbar of thesamesize andmaterial, usingnoexternalmeans? ∗∗ In thebasement of a housethereare threeswitches thatcontrol three light bulbs in the firstfloor.Youareinthebasementandareallowedtogotothefirstflooronlyonce.How doyoufindoutwhichswitchcontrolswhichbulb? ∗∗ Howdoyouwireupalightbulbtothemainsandthreeswitchessothatthelightcanbe switched on at any of the switches and off at any other switch? And for four switches? Nobodywill takeaphysicistseriouslywhoisabletowriteMaxwell’sequationsbutcannotsolve this little problem. ∗∗ 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. ∗∗ 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
liquid electricity, invisible fields and maximum speed 57 Challenge 42 d Challenge 43 s ∗∗ F I G U R E 24 A common playground effect (© Evan Keller). What happened in Figure 24? Why are most of such pictures taken in good weather and with blond children? ∗∗ A PC or a telephone can communicate without wires, by using radio waves. Why are these and other electrical appliances not able to obtain theirpower via radio waves, thus eliminating power cables? ∗∗ Magnetic storage looks far less mysterious if it is visualized. Figure 25 shows how simply with can be done. The method also allows taking films. What happens inside a metal when it is magnetized? The beautiful films at www.youtube.com/watch? v=HzxTqQ40wSU and www.youtube.com/watch?v=LFC6tbbMUaA, taken by Hendryk Richert of Matesy, show how the magnetization regions change when a magnet is approachedtoapieceofmetal.Alsothesefilmshave been made with a simple microscope, using as only help a polarizer and a layer of yttrium iron garnet deposited on glass. ∗∗ Also plants react to magnetic fields. In particular, different magnetic fields yield different growth patterns.The mechanisms, related to the cryptochrome system, are still a subject of research. ∗∗ Magnets can be used to accelerate steel balls.The most famous example is theGaussrifle shown in Figure 26. If the leftmost ball is gently rolled towards the first magnet, the third ball is strongly kicked away.Then the process repeats: the speed increases even more for the fifth, the seventh and the ninth ball. The experiment never fails to surprise whoever Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net
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ChristophSchiller MOTION MOUNTAIN t
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Editiovicesima octava. Proprietassc
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Page 8 and 9: 8 preface PHYSICS: Describing motio
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Page 12 and 13: 12 contents Do we see what exists?
Page 14 and 15: Light, Charges and Brains Inourques
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110 3 what is light? incoming light
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Chapter 4 IMAGES AND THE EYE - OPTI
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Chapter 5 ELECTROMAGNETIC EFFECTS R
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254 7 the story of the brain ∗∗
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Chapter 8 THOUGHT AND LANGUAGE Ref.
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Chapter 10 CLASSICAL PHYSICS IN A N
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362 bibliography 10 Thiseffect hasf
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364 bibliography 37 A discussionof
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366 bibliography M.B. van der Mark
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CREDITS Acknowledgements Many peopl
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NAME INDEX A Abbott A Abbott,T.A.37
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406 subject index A units 328 Aplys
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