# Field Strength Of Pulse

I'm new to the forum, so hello!

How can i calculate the magnetic field strength of the pulse of an air core coil, the number of turns is 20 & the height of the coil is 20mm, not single or multi layered just random wound.
The pulse is 10V & 1Amp, the Inductance is 200uH.

There are several web-based inductance calculators, but they all give lower inductances than you measured. Once the inductance can be calculated approximately, the B field inside the coil can be estimated. You did not provide the coil diameter, which would be useful.
For now, the very approximate B field inside the coil using the solenoid formula is

B =u0 N I/z Tesla, where
u0 = 4 pi x 10-7 Henrys per meter
N = 20 turns
I = 1 amp
z = 0.02 meters

So B = 1.25 x 10-3 Tesla = 12.5 Gauss

One can also use the short coil formula from integrating the Biot-Savart Law yielding

B = (u0/2) N I/R (where R = radius)
= 1.25 x 10-3 Tesla = 12.5 Gauss using R = 1 cm (0.01 meters)

Thanks Bob S,

The coil Radius is 150mm--300mm Diameter, i am very interested to know how the diameter affects the field strength.

I see you mention the B field "inside" the coil, is there a way to calculate the strength of the B pulse at distance away from the coil, say 25mm 50mm 75mm etc?
Thanks Again, your post is very usefull!

Sorry Bob

Edited:

I am missing something,
u0 = 4 pi x 10-7 Henrys per meter, = Permeability of Air------Correct?

Say L Inductance is 300uH or 400uH instead of 200uH how does L fit into the equation?

Last edited:
The integral of the Biot-Savart Law for a circular loop relates the current in a current loop of radius R carrying a currrent I to the magnetic field anywhere along the axis of the loop.

B = (u0/2) I R2/(x2+ R2)1.5
where units are meters, amps and Tesla. x is the distance along the axis from the center of the loop. uo is the permeability of free space.

Use the web calculator http://www.66pacific.com/calculators/coil_calc.aspx
to calculate inductance.