From:   R.G.
Date:
6/22/2001 1:40 PM
Subject:
Units tutorial

one Farad = one coulomb of charge stored per volt. A farad is a huge amount of capacitance, so we'd like something smaller - especially since making capacitors as big as 1F is very hard.

The techie world uses special words for certain units.
1/10th => "deci", as in deciliter, a tenth of a liter
1/100th => "centi", as in centimeter, one hundredth of a meter
1/1000th => milli, as with our friend, the millimeter.

In the science world, we like to use three digits for precision (that specifies things to the nearest 1/1000th) so it is handy to use every third power of ten for units. Therefore, we use 1/1000, 1/1000000, and so on.

Historically, really big capcitors were about 1 micro farad, so that got to being the generally used unit. Even today, we say 10K uF instead of 10 millifarad, which would normally be more in line with the rest of the jargon.

one millionth => micro = 10**-6
one billionth => nano = 10**-9
one trillionth => pico = 10**-12
1/1000 of a trillionth => femto = 10**-15

A pico farad is literally a millionth of a microfarad, so it was once called a mmF, for micro-micro-farad. Nano farads get a lot more use in european electronics than in the US for some reason. In the US, we tend to say "0.001uF" instead of 1nF.

4700pF is equal to 4.7nF, and 0.0047uF.

We do the same thing with resistors, just the other direction. One ohm is a low resistance, so we use
kilo-ohms (1000 ohms) and megohms (one million ohms). Giga-ohms and tera-ohms (10**9 and 10**12) values are not used much, largely because they approach the resistance of air gaps. Switch designers and transformer designers use milli-ohm (1/1000th ohm) numbers for the resistance of things a lot.

R.G.

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