MadSci Network: Physics

Re: What is electric permittivity of free space and how is it calculated?

Date: Mon Mar 2 22:13:00 1998
Posted By: Eric Maass, Operations Manager, semiconductors / communication products
Area of science: Physics
ID: 886795735.Ph

The electrical permittivity of free space uses the symbol of the Greek 
letter epsilon, with a small subset 0.  Permittivity relates to how 
capacitors work.

Anytime you have two conductors, or electrodes, separated by an insulator 
or dielectric, you have a capacitor.  A capacitor can be used to store a 
charge - if you put a positive or negative charge on it either from a 
static electricity generator or from a battery, the charge will be stored 
in the capacitor.  This is because the positive charge on one electrode 
will be attracted to the relative negative charge on the other electrode, 
but will not be able to get through the insulator or dielectric in between.

A capacitor can also be used to filter alternating current signals.  Higher 
frequencies pass through a capacitor easily - when the signal is positive 
on the electrode, it attracts negative charges on the other electrode. When 
the alternating signal changes to negative, it repels the negative charges 
previously attracted. In fact, the "treble" and "bass" tone controls on 
your stereo, CD, or other sounds system probably use capacitors to build 
low pass and high pass filters, using the fact that higher frequencies pass 
through a capacitor more easily than lower frequencies. Treble means higher 
frequencies, bass means lower frequencies are passed through the filter.

The value of the capacitor is the capacitance, and it is related to three 
things: the area of the electrodes in the capacitor, the distance between 
the two electrodes, and the capacitive nature of the dielectric between.

The dielectric, which is an insulator, can be paper, glass, plastic, or 
anything that doesn't conduct electricity - even air or other gases. In 
fact, the insulator can be nothing at all - a vacuum, or free space. If you 
build a capacitor with two electrodes, separated by a space, then pull a 
vacuum so that the space between the electrodes is free space, the 
dielectric is free space, and the equation for the capacitance would be:

     Capacitance = (permittivity of free space) x (Area) / (distance)

           where Area is the area of the electrodes, and
                 distance is the distance between the two electrodes.

If we build this free space capacitor, and know the area of the electrodes, 
and the distance between the electrodes, and then measure the capacitance - 
we can calculate the permittivity of free space.

 If the dielectric is not free space, but air, glass, paper,or something 
else, we define how it works in a capacitor RELATIVE to free space, and 
call that the dielectric the equation becomes:

Capacitance = (dielectric constant)x(permittivity of free space) x Area

Please let me know if you still are confused about this concept.

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