Re: SCIENCE FAIR BOTTLE CAPACITOR ECPERIMENT HELP!!!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Chris the great" <downnessisgoingup-at-hotmail-dot-com> 

Ravi

uF means micro-Farads, wich is a measure of the charge a capacitor can 
store.  Of course, you will probably use nF (nano-Farads) for your 
coil.  Go with saltwater.  Gatorade will not be as conductive and will 
probably cause some other problems.  Also, Gatorade is EXPENSIVE!
Use clear glass, I have heard alot of bad things about coloured glass.

To easily measure capacitance, you can take two routes.  You can get a 
meter that measures capacitance, or more likely, you'll have to do it the 
hard way.  The hard way is hooking up the capacitor to the mains voltage 
and measuring the current that flows with a AC current meter.
Because the capacitor has reactance (resistance to the voltage change of an 
AC current), it will draw current based on the capacitance.  This requires 
a bit of math.

Effective resistance is 1/(6.28 x F x C), where F is the frequency of the 
AC source, usually 60, or 50 if you're in the UK, and C is the capacitnce.
Since we don't know the value of C, we can find it using this formula.
So we have a current measurement, for examples sake, lets say we get 300uA 
(300 micro-amps).
Using Ohm's law, we find the effecitive resistance is V/I, or input voltage 
divided by the current, giving us 400000 ohms if the voltage is 120 volts.
Now we can calculate the capacitance, because we know that 1/6.28 x 60 x C 
is equal to 4000000.
So, take 1, divide it by 400000, then by 60, then by 6.28.  We get 
0.000000006635, or 6.6635 nano-Farads (multiply by 1 billion).  To double 
check, plug in the value into the equation (1/6.28 x 60 x 0.000000006635), 
and we get 400000, the measured resistance.  Take your line voltage, and 
divide it by the calculated resistance.  It should come out to the same 
value as the current that you measured.  If it doesn't, you have made a 
mistake in the math, double check, and try it again.

I have calculated all my bottle capacitors like this, because I haven't 
actually found a capacitance meter.  You should also measure the voltage 
you are putting in, so can make sure your measurements are accurate.

If you don't have a meter that can measure AC current, then you can either 
pick up a cheap one that can measure small currents or make a full wave 
rectifier out of 4 diodes.  This will put out DC for your meter to measure, 
but allow AC to flow accross the capacitor.
I'm sure you can ask a science teacher on making one of these, and you can 
pick up the diodes at radio shack.  They will be black, with a grey or 
white stripe on one end, and are called 'Zener Diodes'.  These ones can 
handle the line voltage, but check with someone at radioshack just to make 
sure.  These should cost just a couple bucks, and you can connect them 
together with some wire scraps.

You will probably find you spend the most time actually calculating out all 
the values before you start building, and this is a good thing.  This means 
that the coil will actually work properly.  Before you start building the 
actual coil itself, post the specs so that we can doublecheck.  I myself 
know how annoying it is to find that the part you just made isn't the right 
value, and having to constantly change things to make the coil funtion 
properly.

There is also a link on the pupman site for the JavaTC calculater, to make 
sure the values are all correct.

Sorry for the long post.


Chris




>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: SCIENCE FAIR BOTTLE CAPACITOR ECPERIMENT HELP!!!
>Date: Wed, 19 Nov 2003 07:47:27 -0700
>
>Original poster: "Nightmare" <nightmare-at-bak.rr-dot-com>
>
>I think i could compare uF ratings with different liquids in the bottle
>caps,(i have no idea what uF is) how do i compare uF ratings, then i could
>conclude which hs the best uF rating.

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