Electromagnet with and without airgap.

[center o bass]

Hello! I'm a bit confused by a phenomenon regarding electromagnets with and without airgaps.

A picture of the kind of electromagnet i have in mind is found here:

http://sci-toys.com/scitoys/scitoys/magnets/calculating/calculating.html

If one has a "loopy" electromagnet without an airgap, the effect of the material will be to boost the magnetic field strength by a certain factor and guide the fieldlines trough the material. This is because the initial field will tend to align the magnetic dipoles inside the material which in turn will create a bigger "current" inside the material.. the current being the source of the magnetic field.

But if one now introduces a little tiny airgap the fieldstrength goes down dramatically.

In the picture of magnetic dipoles, induced volumecurrents and surface currents, what is the physical explanation of this?

 

[AndyNewton]

... if one now introduces a little tiny airgap the fieldstrength goes down dramatically...what is the physical explanation of this?

The easy answer:

By analogy with an electrical circuit.. If you connect a voltage source to a short-circuit then a large electric current flows. If you then introduce a small resistance in the circuit the electrical current goes down dramatically.
The iron core of a toroidal (donut with a hole) electromagnet is equivalent to a magnetic "short circuit". The tiny airgap is equivalent to a magnetic resistance ("magnetic reluctance" is the correct term for this.)

The hard answer:

To fully, and correctly, explain what is going on in the air gap and in the iron core we need to use the two separate concepts of Magnetic Flux Density "B" and Magnetic Field Strength "H". Inside and outside the core those two quantities behave in different ways. Explaining the theory needs far more space than available here. The Wikipedia article "Magnetic Field" will get you started.