MOTION MOUNTAIN

CHALLENGE HINTS AND SOLUTIONS
Page 31
Challenge 1, page 9: Donothesitatetobedemandingandstrict.Thenexteditionofthetextwill
benefitfromit.
Challenge 3, page 16: Theelectricfielddistortstheflametowardsandagainstthecomb.Anphotographoftheeffect
isshowninFigure171.Avideoofasimilareffectinstrongerfieldsisfound
atwww.youtube.com/watch?v=a7_8Gc_Llr8.
Challenge 4, page 20: The liquid drops have to detach from the flow exactly inside the metal
counter-electrodes.Thereisalwaysatinychargesomewhereonthemetalstructures(duetocosmicrays,rubbing,previouscharging,etc.).InFigure170,thisinitialchargeisthepositivecharge
drawn on the lower left and upper right metal structure. When the water droplets form, they
get a charge that is opposite to that of the metal surrounding the region where they form. The
negativedroplets fall intotheothermetal structure.Through thenegativecharge accumulating
there,thepositivechargeinthefirststructureincreases.Whenthechargeonthemetalstructure
increases, the charge separation in the droplets is more efficient. In other words, water droplet
formationinsidethemetalstructuresamplifiesanyinitialcharge.Afterawhile,thechargevalue
andtheassociatedvoltagearesohighthatitleadstoaloudbang(ifeverything isdry,including
the air.) Then the process starts again. In fact, a vaguely similar charge separation mechanism
leads to cloud charging and to lightning.If you want to build a Kelvin generator at home,have
alookatthede.wikipedia.org/wiki/Kelvin-Generator webpage,orsearchforotherinternetsite
onthetopic.
Toavoidthesparksinthefueltanksofitscars,Opelsimplyearthedthemetaltubeatthetank
inlet;theyhadforgotten toensureelectric contactbetween thetubeandtherestofthecar.
The explosion of fuel can also occur if you pour fuel into your car from a metal container.
Several times, fuel thieves were ‘punished’ by an explosion triggered by static electricity when
theytried topourstolenfuelintotheirowncar.
Oneveryairportyoucanseehowtheproblemisavoided:beforeevenattachingthefueltube
toanaeroplane,theworkerattachesaconductingcabletoconnectthetruck(orthetank)tothe
aeroplane.
Challenge 5, page 21: We lookatthetwo sparks through a rapidly rotating mirror. In thisway,
small timing differences lead to position differences of the two sparks. In the 19th century, the
speedvaluesmeasuredinthiswayvariedbetween6000 km/sandwayover100000 km/s,because
thespeed depends ontheeffective capacitanceandinductanceofwireand set-up.Only if these
effectscanbeneglectedisthemeasuredspeedthesameasthatoflightinvacuum,namelyaround
300000 km/s.Inmoderncables,thespeedisoftenaroundathirdofthisvalue.
Challenge 6, page 21: Alotofnoiseappearedwhilethemetalpendulumbangedwildlybetween
thetwofixed bells.
Challenge 8, page 25: No.
Challenge 9, page 26: Thefieldatadistanceof1 mfromanelectron is1.4 nV/m.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net