# Finding capacitance

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#### Cia

##### New Member
Hi all, i've got a question - What formula would I use to find the capacitance required in series with a resistance of 100 ohms so that the phase difference of the circuit is 60 degrees when the frequency is 40 Hz.

Much appreciated Hi Cia,

Do you want to take your output from across the capacitor (voltage) or
across the resistor (voltage), or as the current through the network?

I believe this is correct: C= 1⁄(200Π(tan(60))
Xc= 1⁄2ΠfC and the phase of Z= tan(Xc/R)

Hello again,

If it really is the voltage across the capacitor as output and because 60 degrees
is one of those 'special' phase angles the formula reduces to:

K=F*R*C
where
F is the frequency in Hertz
R is the resistance in ohms
C is the capacitance in Farads
and K is a constant:
K=sqrt(3)/(2*pi) which is approx equal to:0.2756644477

so to solve for R we would get:
R=K/(F*C)

or to solve for C we would get:
C=K/(F*R)

For example:
with C=1000uf, R would come out to about 6.892 ohms.

If C=100uf then R would come out to about 68.92 ohms, and if
C=10uf then R would come out to about 689.2 ohms.

Last edited:
Thanks MrAl and Gary B, your answers really helped me out. Have a great christmas!

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