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Author Topic: coil dimensions puzzle  (Read 2126 times)

iolantham

• First timers
• Posts: 4
coil dimensions puzzle
« on: 23/06/2007 03:33:44 »
Hello Physics enthusiast!

Here's my puzzle: I need to find the dimensions of the coil which will give me the magnetic field strength H=100 A/m.
A=diameter of the coil
B=length of the coil
N=# of turns
I=current
M=magnetization=300 A/m
f=frequency=40*10^6 Hz
L=inductance
ρ=16/6 turns/mm=8000/3 turns/m
Vcoil=300V(maximum value)
H=N*I/B-M
I=Vcoil/(2*π*f*L)
L=2*A^2*N^2/(3*A+9*B) (this is the only place where SI units are not used: A and B are in inches and L is in μH!!!)
N=ρ*B
=>(3*A+9*B)/A^2*N^2=1787217.154
For A=.1 inches=2.54 mm=>B=.0005 inches=.00197 mm!!!
That means I have no length!
Are my formulas wrong for this circuit?

syhprum

• Neilep Level Member
• Posts: 3765
• Thanked: 14 times
coil dimensions puzzle
« Reply #1 on: 23/06/2007 06:54:07 »
You seem to have made an error copying out the formula for the inductance of a coil, you used  2A^2*N^2/(3A+9B) it should have been .2A^2*N^2/(3A+9B).

I apologise for the use of olde worlde non S.I units but I was quoting from my prewar boyhood recollections before S.I became established

The Naked Scientists Forum

coil dimensions puzzle
« Reply #1 on: 23/06/2007 06:54:07 »