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MOTION MOUNTAIN

LIGHT, CHARGES AND BRAINS - Motion Mountain

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64 1 electricity and fields<br />

battery<br />

suspending<br />

wire<br />

N<br />

S<br />

mercury<br />

Challenge 50 s<br />

Challenge 51 s<br />

Challenge 52 s<br />

Ref. 32<br />

Challenge 53 s<br />

F I G U R E 32 A unipolar motor.<br />

∗∗<br />

F I G U R E 33 The simplest motor (© Stefan<br />

Kluge).<br />

The shortest light pulse produced so far had a duration of100 as. To how many<br />

wavelengths of green light would that correspond?<br />

∗∗<br />

How long can batteries last? At Oxford University, in Clarendon Hall, visitors can watch<br />

a battery-operatedelectricbellthatisringingsince1840.Thetwobatteries,twoZamboni<br />

piles,produceahigh voltage and low current, sufficient to keep the bell ringing. Several<br />

other similar devices, using Zamboni piles, have worked in Italy with the same batteries<br />

for over 100 years.<br />

∗∗<br />

Why do we often see shadows of houses and shadows of trees, but never shadows of the<br />

electrical cables hanging over streets?<br />

∗∗<br />

How would you measure the speed of the tip of a lightning bolt? What range of values<br />

do you expect?<br />

∗∗<br />

One of the simplest possible electric motors was discovered by Faraday in 1831. A magnet<br />

suspended in mercury will start to turn around its axis if a current flows through it. (See<br />

Figure 32.) In addition, when the magnet is forced to turn, the device (often also called<br />

Barlow’s wheel) also works as a current generator; people have even tried to generate<br />

domestic current with such a system! Can you explain how it works?<br />

The modern version of this motor makes use of a battery, a wire, a conductive<br />

samarium–cobalt magnet and a screw.The result is shown in Figure 33.<br />

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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