# rearrange equation to find 2 unknowns

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• May 26th 2009, 07:19 PM
tommoturbo
rearrange equation to find 2 unknowns
can some one help with this formula please i need to find 2 unknowns
A area and d distance

Formula is

C = Er*Eo*A / d

where
C= 100x10 power-12
Er=12
Eo= 8.85x10power-12

i can isolate d/A to equal Er*Eo/C which makes d/A = 1.062 ohms

and get d= Er*Eo*A / C

but brain wont go any further

any help much appreciated

thanks Ian
• May 26th 2009, 07:37 PM
pickslides
$A= \frac{Cd}{E_rE_0}$

and

$d = \frac{AE_rE_0}{C}$

do you have the units for $E_r$, $E_0$ and $C$ ?
• May 26th 2009, 07:42 PM
tommoturbo
hello there that was a quick reply thanks!

the units are
C= 100pico farads
Er is relative permittivity of 12 (ratio no units)
Eo is relative permittivity of air 8.85x 10power-12
A in Meters ie but very small area
d in metres but likely around 1mm or less

as its to do with capacitors

thanks Ian
• May 26th 2009, 07:56 PM
pickslides
Sorry Ian, a bit out of my league.... (Worried)
• May 26th 2009, 08:00 PM
tommoturbo
(Surprised)

thanks for having a go, my tutur told me to make d the subject and use that value in another equation but the question dosnt have a value for the area so im really not sure how to do it

Regards Ian
• May 26th 2009, 08:06 PM
pickslides
You still have 2 unknowns so the best you can do is make a relationship between the 2 values.

substituting in the values given $A = \frac{513}{500}d$
• May 26th 2009, 08:16 PM
tommoturbo
Thanks for taking the time m8

back to thr drawing board i think

Cheers Ian