# The Naked Scientists Forum

### Author Topic: formula for magnetic field strength, H  (Read 6102 times)

#### iolantham

• First timers
• Posts: 4
##### formula for magnetic field strength, H
« on: 19/06/2007 03:16:15 »
Hello!

I have a coil in an AC circuit. I tried to apply these formulas to get H, but it seems there is a problem w/ my formula (H should be proportional to N, no. of turns).
H=N I/l-M, I=current, l=length, M=magnetization
I=Vcoil/(2 π f L), f=frequency, L=inductance
L (inductance for an air coil)=K μ0 π r^2 N^2/l, K=Nagaoka coeff, 0<K<1, r=radius of the coil
In the end, substituting L into I and then I into H I get H proportional to 1/N !!!!!(H=Vcoil/(2 π f K μ0 π r^2 N) -M, which seems to contradict my first formula for H.

#### syhprum

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##### formula for magnetic field strength, H
« Reply #1 on: 19/06/2007 06:14:07 »
When I was a boy and used to construct crystal sets I used a formula from the twenties for computing the inductance of a coil in μH .2A^2*N^2/(3A+9B) A=diameter, B=Length, N=number of turns (all in inches), seemed to work OK .
The formula for computing the resonant wavelength was 1885*(L*C)^.5, L=μH, C=μF, How quaint these prewar units seem today!
« Last Edit: 19/06/2007 18:41:26 by syhprum »

#### The Naked Scientists Forum

##### formula for magnetic field strength, H
« Reply #1 on: 19/06/2007 06:14:07 »