Xref: utzoo sci.electronics:6388 sci.physics:8381
Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!fluke!strong
From: strong@tc.fluke.COM (Norm Strong)
Newsgroups: sci.electronics,sci.physics
Subject: Re: HV Cap Fun!
Keywords: capacitor,energy,paradox
Message-ID: <8742@fluke.COM>
Date: 1 Jun 89 16:23:18 GMT
References: <4924@m2c.M2C.ORG> <3806@mit-amt> <20772@quacky.mips.COM> <8739@fluke.COM>
Sender: news@tc.fluke.COM
Organization: John Fluke Mfg. Co., Inc., Everett, WA
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In article <8739@fluke.COM> strong@tc.fluke.COM (Norm Strong) writes:
}In article <20772@quacky.mips.COM> vaso@mips.COM (Vaso Bovan) writes:
}}A Paradox of Capacitor Energy Storage
}}
}}I've heard several competing answers to this paradox. None is entirely
}}satisfactory:
}}
}}Consider an ideal 2uF (for computational ease) capacitor charged by a 10 volt 
}}source. Eventually, the energy stored is (1/2)*CV^2=100 joules.
}
}Well. . . 100 microjoules.  But it's the principle that counts.  Right?
}
}}Consider the capacitor to be isolated from the voltage source, and then
}}directly shorted across an identical (ideal) capacitor. Eventually, the
}}voltage across each capacitor will be 5V. Now, there are two equally
}}charged capacitors, each storing (1/2)*CV^2= 25 joules, for a total of
}}of 50 joules.  What happened to the other 50 joules ?
}
}The extra energy is dissipated in the form of heat when the current flows
}though the series resistance of the capacitor.  If the series resistance
}of the capacitor is negligible, it will be dissipated in the form of a
}bright flash accompanied by a loud bang (but you won't notice it until the
}values get appreciably higher than 10E-4 joules.)
}At about 1000 joules, the flash will eat off the end of the wire you use
}to connect the capacitors together, and the bang will scare the hell out
}of you.    :-)

[Change that to 1E-4}

It also might be of interest to note that electrical discharge is used for
some types of machining.  It's possible to machine a corkscrew shaped hole
in a solid block of metal using this method.  Just try it any other way!
-- 

Norm   (strong@tc.fluke.com)