Re: Jurassic Park

[bdow@ee.ubc.ca (Bruce Dow)]

In article <1993Jul7.164152.30229@ns1.cc.lehigh.edu> dg04@ns1.cc.lehigh.edu (DAVID GRUMBINE) writes:
>
>Actually, it's more to save the power supply.  An electric fence has almost
>no resistance (just plain conductors) so keeping the power supply on for
>any length of time would either fry the supply or melt all the wire in the
>fence.
>

Well! Finally a physicist gets in on it!! You are right in that it is
to save the power supply (I said 'to keep costs down' for simplicity).
But you are wrong about the electric fence having almost no resistance!
The current does not flow along the fence! The voltage is applied to the
fence and flows to ground when something closes the path. The reason
pulsing the high voltage saves money is that the wooden posts are
not perfect insulators so there would be some leakage current.

>>I remember the stick in the movie looked quite dry (although I
>>guess that was a high-humidity environment). On the cattle
>>fence the stick would have to be moist and touch the high-voltage
>>wire *and* ground simultaneously in order to get zapped. But with
>
>With real high voltages (10kV isn't really THAT high) being wet or dry
>wouldn't make any difference.
>

Hmmm... True again. But I was only speaking of the cattle fences I saw.
A dry stick would have about the same resistance as the fence posts (there
were no insulators between the wires and the posts). And the voltage was
considerably less than 10kV. Dry wood isn't a bad insulator (it was used
in early automobile ignition systems!) If a dry stick would get zapped
by the cattle fence then the fence's own posts would be getting
continually zapped, too (whatever the technical definition of
'zapped' is!!!)

>>the short pulse-length they use, the zap would not be visible.
>
>Ahh, no.  Ever get shocked by normal house current?  Those pulses are only
>1/60 second.  Pretty short.  But you can see spark if you short it out!
>Mighty big spark too, I can add from experiance!
>

Me too! But I neglected to mention that these were very short duration
pulses occuring about once every second or two...

>>[....]
>>I guess you have to be like a kid and suspend disbelief
>>in order to really enjoy some movies.
>
>Oh well.  Good movie anyway, even with these little oversights.  :-)
>

No kidding! I just about crapped myself when the T-Rex was
chasing them in the jeep!  :-)

Bruce


-- 

Bruce Paul Dow,  undergraduate student, (third year)
Electrical Engineering (Computer Engineering Option)
The University of British Columbia (Vancouver, B.C.)
Member of Team 10, Developers of:  __Mail_System_X__