# Calculating Field strength of a air core solenoid

I want to build a magnetizer for charging / magnetizing small alnico magnets for that I am thinking to discharge a cap into a air core solenoid.

The magnet to be charged would be kept within this solenoid.

The magnet would be of cylindrical geometry and 5mm dia and 10mm length, after searching on the web I have found that alnico needs a field strength of 3000 Oersted or around 239 kA/m.

I find that Oersted is related to A/mor Ampere/meter as both of these are units of field intensity but I don't know how to relate oersted to ampere-turns.

To generate this field intensity I would like to know how many turns to wind and how much ampere's to dump into the coil.

Any clues would be of great help.

I don't know how to relate oersted to ampere-turns

Or, muliply the oersted value by 1000 and divide by 4$\pi$ to get amps per metre (aka ampere-turns per metre).

Any clues would be of great help

To get 3000 oersted requires a lot of turns and a lot of amps but I think you should be armed with the knowledge now.

Here's another hint - the H field is "per metre" and the length of your solenoid is 10mm so the amp-turns you need are 0.01 x 239,000 = 2390 ampere-turns so as a couple of examples, that's either 1 amp and 2390 turns (for every centimetre) or 10 A and 239 turns per cm.

I would also consider that the solenoid length should be three times the length of the material to be magnetized to ensure that the resulting flux density is constant along the length; at the open ends of a solenoid the flux can be significantly reduced compared to the centre.

• Thanks for the answer, Ampere-turns is the unit for MMF right? Where as A/m is unit for Field Intensity. So is A/m or Ampere-turns/m the same thing I mean are both units of Field Intensity? Dec 2 '16 at 17:14
• Would the material also affect the MMF to be given I mean after I insert the Alnico magnet into the coil wouldn't the permeabilty of the coil change? Dec 2 '16 at 17:20
• Yes, the "At" is MMF and, H is MMF. H is calculated as ampere-turns per metre but the "turn" bit is dropped when referring to this or that field intensity because it's impossible to distinguish bettwen 1 amp and ten turns or ten amps and one turn. Dec 2 '16 at 17:21
• Materials are irrelevent except for increading flux density (which is a by product of H and permeability of the material inserted. Dec 2 '16 at 17:22
• H is MMF per metre for sure. The picture shows the relationship between oersted and A/m as I thought you also required. Dec 2 '16 at 17:33