From: SFPhysics@aol.com
Date: Sun Apr 16 2000 - 01:55:32 PDT
From: SFPhysics@aol.com Message-ID: <a1.40b3da4.262ada04@aol.com> Date: Sun, 16 Apr 2000 04:55:32 EDT Subject: Electrical Safety/was incandescent pickle
> Subject: cfgi pickle plug
> From: "MC elover" <mcelover@yahoo.com>
> Date: Fri, 14 Apr 2000 08:57:04 -0700 (PDT
> hello
> Don's suggestion for using a CFGI outlet for the
> sparkly pickle demo is great. But how does the>
> circuit know to shut off for a person and not for a
> pickle? Even with a little vineagar on my hands, I
> would assume that I have more resistance than a pickle
> (but less flavor and not as crispy)
> That's the best tasting physics teacher I ever heard.
>--eric
>>
Eric, et al.:
The Ground Fault Interrupter is a circuit breaker that detects how much
current is going into and out off a socket. Inside is a very sensitive
differential amplifier that senses current. If 1 Ampere goes out one wire of
the plug then exactly 1 Ampere should be coming back through the other wire
of the plug. If a human accidentally touches a live circuit where the human
may send some current through another path, the differential amplifier shows
that maybe 1.1 Amperes is going out but only 1.0 Amperes is coming back. The
amplifier triggers the breaker to open the circuit and remove the power. The
human is singed but saved.
The GFI WILL NOT PROTECT you if you are in the path *between* the two prongs
of a plug in the circuit. It only cuts out if you are standing in a puddle
of water and some of the current goes elsewhere rather than back into the
plug.
For years my students made clear Plexiglas hot dog cookers with safety
interlocks where the electrodes could not be touched until the cord was
pulled from the unit and the power disconnected. We carbonized a few hot
dogs but never even tingled a human.
I am not sure I would even want to do a demo that the students might repeat
without supervision or understanding. Having nails on the ends of a zip cord
scares even me (who puts 300,000 Volts through myself to light a fluorescent
lamp I hold in the air above me for a Tesla Demo). Human skin resistance
through dry skin is enough to keep you from getting killed instantly but add
just a drop of electrolyte (vinegar, salt water, etc.) and skin resistance
drops like a rock thus letting current through the sensitive parts of your
body jump dramatically. Most home and lab wall circuits are fused to open
after 15 Amperes while death occurs at currents of only 0.1 Amperes under the
right conditions.
Lastly, if you absolutely must do the pickle demo, a good idea would also be
to use a large double pole singe throw 20 Ampere industrial pushbutton where
you could press down on the button activativing the electrodes, release to
deactivate. This would be better than inserting and removing a plug
especially in haste with an erupting or exploding pickle in the vicinity. A
ballast resistor would be another good design point.
Al Sefl
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