#### Ghina Bayyat

Joined Mar 11, 2018
135
hi
i need to ask why are there two equations for calculating the capacitance of the capacitors ? in some books c = 10 / 2π f Xc and in other books it is c = 1 / rπ f Xc
which one is right ?

#### AlbertHall

Joined Jun 4, 2014
11,316
hi
i need to ask why are there two equations for calculating the capacitance of the capacitors ? in some books c = 10 / 2π f Xc and in other books it is c = 1 / r π f Xc
which one is right ?
Neither.
Xc = 1 / (2 π f C)
C = 1 / (2 π f Xc)

#### dl324

Joined Mar 30, 2015
12,871
hi
i need to ask why are there two equations for calculating the capacitance of the capacitors ? in some books c = 10 / 2π f Xc and in other books it is c = 1 / rπ f Xc
which one is right ?
Neither.
$$\small X_C = \frac{1}{\omega C} = \frac{1}{2\pi fC}$$

#### Jony130

Joined Feb 17, 2009
5,230
I sometimes use this approximation 1/(2π) ≈ 0.16 hence Xc = 0.16/(F*C)

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#### Ghina Bayyat

Joined Mar 11, 2018
135
so C = 1 / 2π f Xc is the right one

#### thedoc8

Joined Nov 28, 2012
153
Yes that is correct.

#### WBahn

Joined Mar 31, 2012
26,398
so C = 1 / 2π f Xc is the right one
What you mean is correct, what you wrote is not.

It is pretty universal that multiplication and division have the same precedence and are performed left-to-right, so what you wrote is actually

C = (1 / 2)·π f Xc = (π f Xc)/2

What you should have written is

C = 1 / (2π f Xc)

Sloppiness in things like this WILL get you in trouble sooner or later. Either because you will tend to do the same sloppy things when you put the formula into a spreadsheet or a program, or because you will cause miscommunication with whoever later reads it (which might even be you).

#### WBahn

Joined Mar 31, 2012
26,398
hi
i need to ask why are there two equations for calculating the capacitance of the capacitors ? in some books c = 10 / 2π f Xc and in other books it is c = 1 / rπ f Xc
which one is right ?
What is 'r' in the second one?

Which books give each of those? What is the context? Not that I can imagine any context in which either is correct.