Generating a magnetic field with an electromagnet
Probably the best way to understand what's going on is to use an electrical circuit analogy again.

The MMF is like a voltage source, and is the number of turns in your coil times the current in the coil.

The permeability of your core is like a resistor connected to your MMF source.  It's high permeability so just like a low value resistor.  You want a high magnetic flux, so your voltage source wants to cause a high current to flow in the resistor.

Now, if your magnetic core were a closed path like a toroid you would be all set.

However, you have a break in your magnetic circuit path.  This is like an additional resistance in series with the small resistor you have from the ferrite core.

This "air gap" resistance analog is very much larger than your ferrite resistance analog.  So no matter how high a permeability your ferrite has your MMF voltage source will not be able to drive a large current in it.  (You won't get a large magnetic flux.)  See the schematic below.  The resistor values are arbitrary, just to illustrate the point.

So with a large air gap the only way to increase the magnetic flux is to increase the number of turns, N and/or the current in the coil.







simulate this circuit &ndash; Schematic created using CircuitLab