MOTION MOUNTAIN

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32 1 electricity and fields TA B L E 5 Some observed electric current values. Observation Current Page 50 Smallest current ever measured (for one 3 aA moving electron) Human nerve signals 20 µA Lethal current for humans as low as20 mA, typically 100 mA Current drawn by a train engine 600 A Current in a lightning bolt 10 to100 kA Highest current produced by humans 20 MA Current inside the Earth, at the origin of its c.100 MA magnetic field Maximum possible current in nature (cor- 1.5 YA rected Planck electric currente√c 5 /4ħG ) TA B L E 6 Some sensors for electrical current. Measurement Sensor Range Conventional 20 euro multimeter voltage drop over resistor up toc.3A Feeling threshold human nerve felt from0.1 mA upwards Reversible muscle contraction without danger human nerve up to10 mA over long times, or up to200 mA for at most10 ms Rhythm change human heart heart stops when about 20 mA flow through it Strong muscle contraction with some damage human nerve up to100 mA over long times, or up to1 A for at most200 ms Smoke emission, strong burns human flesh from1 A Fire trees from1 kA Electric eelElectrophorus electricus bult-in up to1 A and500 V themedicaldoctorLuigiGalvani (b. 1737 Bologna, d. 1798 Bologna) discovered thatelectricalcurrentmakesthemusclesofadeadanimalcontract.Thefamousfirstexperiment usedfroglegs:whenelectricitywasappliedtothem,theytwitched violently. Subsequent investigations confirmed that all nerves make use of electrical signals. Using electricity, one can make fresh corpses move, for example. Nerves are the ‘control wires’ of animals. We will explore nerves in more detail below. Being electrically controlled, all mammals can sense strong electric fields. Humans can sense fields as low as10 kV/m, when hair stands on end. In contrast, several animals can sense much weaker electric (and magnetic) fields. Sharks, for example, can detectfieldsdownto0.5µV/musingspecialsensors,theAmpullaeofLorenzini,whichare found around their mouth. Sharks use them to detect the field created by prey moving 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 33 TA B L E 7 Searches for magnetic monopoles, i.e., for magnetic charges, in over 140 experiments. Search Magnetic charge Smallest magnetic charge suggested by quantum theory g= h e =eZ 0=4.1 pWb 2α Search in minerals, from mountains to the deep ocean none, only dipoles Ref. 17 Search in meteorites and moon minerals none, only dipoles Ref. 17 Search in cosmic rays none (one false alarm in the 1970s), only dipoles Ref. 17 Search with particle accelerators none, only dipoles Ref. 17 Ref. 15 Ref. 16 Challenge 19 r Page 104 in water; this allows them to catch their prey even in the dark. The muscles in living prey generate electric fields. Various water animals have developed electric field sensors to detect prey in water which is too muddy to see through. The salamander and the platypus (Ornithorhyncus anatinus), the famous duck-billed mammal, can also sense electric fields; but they achieve only sensitivities of the order of mV/m. Numerous fish, the so-called strongly and weakly-electric fish, even generate electric fields in order to achieve better prey detection.* This approach is used, for example, by the elephantnose fish (Gnathonemuspetersii).The achieved sensitivity is below2 mV/m. Infact, various electric fish usetime-varyingelectricdipole fields to communicate! They tell each other their species, their sex, their identity, and communicate about courtship, aggression,appeasementand dangers. The frequencies they use are in therangebetweenafewand200 Hz,andthefieldsaredipolefieldscreatedbetweenthe anteriorandposteriorsectionsoftheirbodies. No landanimal hasspecial sensorsforelectricfields,because any electricfieldin air is strongly damped when it encounters a water-filled animal body.** Indeed, the usual atmospherehasalow, vertical electric field of around100 V/m; inside the human body this field is damped to the µV/m range, which is far less than an animal’s internal electric fields. In other words, humans do not have sensors for low electric fields because they are land animals. (Do humans have the ability to sense electric fields in water? Nobody seems to know.) However, there a few exceptions. You might know that some older people can sense approaching thunderstorms in their joints.This is due the coincidence between the electromagnetic field frequency emitted by thunderclouds – around100 kHz – and the resonant frequency of nerve cell membranes. The water content of the human body also means that the electric fields in air that are found in nature are rarely dangerous to humans. But whenever humans consciously sense electric fields, such as when high voltage makes their hair stand on end, the situation is potentially dangerous. The high impedance of air also means that, in the case of time-varying electromagnetic fields, humans are much more prone to be affected by the magnetic component thanbytheelectriccomponent. *Ittookuntiltheyear2000fortechnologytomakeuseofthesameeffect.Nowadays,airbagsensorsincars often useelectric fieldsto sensewhetherthe person sitting in the seatis a child or an adult, thuschanging thewaythattheairbag behavesinanaccident. **Thoughafewlandanimals thatswimalot underwaterhaveelectricfieldsensors. Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net
Page 1: ChristophSchiller MOTION MOUNTAIN t
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Page 12 and 13: 12 contents Do we see what exists?
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Chapter 3 WHAT IS LIGHT? Ref. 57 Pa
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96 3 what is light? Electric field
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98 3 what is light? spark transmitt
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100 3 what is light? Ref. 59 Page 1
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102 3 what is light? primary infrar
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106 3 what is light? Fre - Waveleng
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108 3 what is light? Challenge 110
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110 3 what is light? incoming light
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112 3 what is light? Ref. 70 Challe
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114 3 what is light? F I G U R E 69
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116 3 what is light? light Ref. 72
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Chapter 4 IMAGES AND THE EYE - OPTI
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142 4 images and the eye - optics 2
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Chapter 5 ELECTROMAGNETIC EFFECTS R
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236 7 the story of the brain F I G
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238 7 the story of the brain Vol. V
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240 7 the story of the brain TA B L
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242 7 the story of the brain consci
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244 7 the story of the brain Ref. 2
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246 7 the story of the brain F I G
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248 7 the story of the brain Vol. I
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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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258 8 thought and language Challeng
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260 8 thought and language Vol. VI,
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262 8 thought and language F I G U
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264 8 thought and language Challeng
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266 8 thought and language Ref. 241
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268 8 thought and language Vol. IV,
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270 8 thought and language . . 0 1
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272 8 thought and language Ref. 251
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274 8 thought and language ⊳ Ever
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Chapter 10 CLASSICAL PHYSICS IN A N
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328 a units, measurements and const
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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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368 bibliography Vol. I, page 96 19
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384 bibliography and 304. 260 For a
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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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402 name index S Sheldon Sheldon, E
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404 name index Z Zybin Motion Mount
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406 subject index A units 328 Aplys
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408 subject index C classical class
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410 subject index E Earth Earth age
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412 subject index F finite finite 2
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414 subject index I infrared photog
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424 subject index T table d’Unit
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426 subject index W Wien’s Wien
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