# Reversing an algebra question

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• Jun 25th 2010, 02:05 PM
ChrisYard
Reversing an algebra question
Hi all,
Please be kind to me :-)

I have this equation,

http://upload.wikimedia.org/math/c/5...e959b8be5c.png

where f = Frequency
L = Inductance
C = Capacitance

Now the problem is I know what f & C is. F = 60kHZ and C = 840pF
Yes this is for an electronics project, I can not work out what L should be, and I have forgotten everything I was learnt when I was at school, many many moons ago !

Please could someone remind me how to do it

Thanks in advance

Chris Yard
• Jun 25th 2010, 02:18 PM
mr fantastic
Quote:

Originally Posted by ChrisYard
Hi all,
Please be kind to me :-)

I have this equation,

http://upload.wikimedia.org/math/c/5...e959b8be5c.png

where f = Frequency
L = Inductance
C = Capacitance

Now the problem is I know what f & C is. F = 60kHZ and C = 840pF
Yes this is for an electronics project, I can not work out what L should be, and I have forgotten everything I was learnt when I was at school, many many moons ago !

Please could someone remind me how to do it

Thanks in advance

Chris Yard

$f^2 = \frac{1}{4 \pi^2 LC} \Rightarrow \frac{1}{f^2} = 4 \pi^2 LC \Rightarrow \frac{1}{4 \pi^2 C f^2} = L$.
• Jun 25th 2010, 02:23 PM
ChrisYard
Quote:

Originally Posted by mr fantastic
$f^2 = \frac{1}{4 \pi^2 LC} \Rightarrow \frac{1}{f^2} = 4 \pi^2 LC \Rightarrow \frac{1}{4 \pi^2 C f^2} = L$.

Thanks mr fantastic, now for some one that is looking through mud !!! sorry for being a pain can I have a step by step ????
• Jun 25th 2010, 02:29 PM
mr fantastic
Quote:

Originally Posted by ChrisYard
Thanks mr fantastic, now for some one that is looking through mud !!! sorry for being a pain can I have a step by step ????

I have given the algebra step-by-step! Now plug in the numbers and press buttons on a calculator.
• Jun 25th 2010, 02:38 PM
ChrisYard
Sorry Mr fantastic,

$
f^2 = \frac{1}{4 \pi^2 LC} \Rightarrow \frac{1}{f^2} = 4 \pi^2 LC \Rightarrow \frac{1}{4 \pi^2 C f^2} = L.
$

So

$
\frac{1}{25.13274122871835 * 840 * 60000^2} = 0.0000000000000131577.
$

????
• Jun 25th 2010, 02:43 PM
mr fantastic
Quote:

Originally Posted by ChrisYard
Sorry Mr fantastic,

$
f^2 = \frac{1}{4 \pi^2 LC} \Rightarrow \frac{1}{f^2} = 4 \pi^2 LC \Rightarrow \frac{1}{4 \pi^2 C f^2} = L.
$

So

$
\frac{1}{25.13274122871835 * 840 * 60000^2} = 0.0000000000000131577.
$

????

Do you understand what the p in 840 pF means?
• Jun 25th 2010, 02:49 PM
ChrisYard
Opps sorry, I think I am going to bed and try again tomorrow !!!

pico = 0.000000000001

$
\frac{1}{25.13274122871835 * 0.00000000000840 * 60000^2} = 1.31576507185760011454
$

so that would mean L = 1.315uH