# How do you calculate

#### Jonathan

the self-capacitance and self-inductance of a coil? Are there equations for these where I won't need to know the number of turns? Since I think #2 is no, are there meters I can buy to measure this, or can I use the measurements from a volt/resistence meter I already have?

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#### zare

the self-inductivity is given by :

L = - (dI/dt)*(1/Us)

where dI/dt is speed of current variation (dI is current variation, dt is time of variation), and Us is sefinductivity voltage.

#### Jonathan

Thanks for repling! How do I know the self-inductivity voltage? Any ideas about self-capacitance? L seems like it should be proportional to the # of turns or the length of wire or something, why not?

#### zare

oi couldnt figure the easier way for determining selfinductivity if you already have em. coil, so here it is :

we need basic three values about coil, it's current, voltage, and resistance. i believe you already know voltage, so you can measure the resistance of coil with normal ohmmeter, and then get current with I = U / R.

we need to know number of turns too. use the formula R = ro * (l/S), where ro is specific resistance (you can find those values on the internet for every material, for copper is 0,0175), l is length of wire and s is surface (in mm2). then you can use simple geometry to determine number of turns. you need to know the surface value for that. N = lair / 2*sqrt(S/3.14). lair is lenght of coil, not of wire, and you can use normal meter to measure it.

in my previous post i gave that formula. Us is selfinductivity, and L is inductivity. Us is voltage that appears when magnetic flux changes inside the coil as direct effect of current change trough coil. so we need to know inductivity to determine selfinductivity.

Us = -L*(dI/dt)

inductivity : L = N^2 / Rm . we already calculated number of turns so onlything that's left is Rm, the electromagnetic resistance.

Rm = theta / fi

theta = I * N , the magnetomotoric force. we have both of values required, the current trough coil and number of turns

fi = B * S, the magnetic flux. note that S here isnt same as surface of the wire, it's the surface of electromagnetic pole. you can determine it by measuring the radius of the coil, and then using r^2*pi.

B = [mi-zero]*[mi-relative]*(I*N)/l , the field strenght. [mi-zero] is permeability of free space constant, and it equals 1,257e-6 (0,000001257). [mi-relative] is permeability of core. if your coil doesnt have iron or other core beneath it, then youre using air coil, and [mi-relative] is 1. for other materials refer to this URL : http://www.oz.net/~coilgun/theory/materials.htm. [Broken]

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#### Jonathan

That was a little over my head and more work than I was expecting. I'm thinking maybe I should just buy a digital inductance/capacitance meter...

#### zare

it's not that hard. if you post here all specifications you have on your system, i'd be happy to do the math for you.

#### Jonathan

I don't yet have a particular system in mind. When I do, I will PM you, thanks.

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