A simple question.

[broli]

I have had this simple question on my mind for while without being able to conduct the actual experiment. So I thought maybe someone who already did could enlighten me.

The question is simple. In the picture below you see an iron core with a coil wrapped around one side making a constant magnetic field due to the DC source. Now the other side has an air gap. My question is if I take another piece of iron core and go through the air gap with it back and forth will this change the FLUX THROUGH THE COIL? My assumption tells me no but I'm not sure about this. According to my reasoning the flux through the coil cannot increase as the H field produced by the coil and the M term will remain the same regardless if the air gap is closed or opened.

Please let me know your view.

[TinselKoala]

Actually, it depends on whether the core material is already saturated by the powered coil or not. If it isn't, then what you have drawn is a basic "permeammeter", which may be used to determine the magnetic permeability of the inserted piece, by watching how the inductance of the coil changes when the piece is inserted and withdrawn.
Of course, if the core is saturated by the flux from the coil, you won't be able to detect these changes.
I think.

[broli]

Your last line is a good one  .

So in the case it's not saturated why exactly will the flux change? Doesn't the part that goes only through and near the ends of the coil matter? If I make changes in the magnetic current elsewhere wouldn't the coil flux see no difference. Either that or I'm misunderstanding magnetization.

Thanks for your answer.

[gyulasun]

Hi,

The flux changes because the created magnetic poles in the C core will be able to close via less reluctance then via the air gap.  Remember, a magnet is always stronger when you use both of its poles, see pot or horse shoe magnets.

Edit:  nevertheless, a test would be the correct answer...   

[CARN0T]

Hi, Broli,

Looking under "magnetic circuits" in a textbook provides your answer.  For example, this is from page 130 of "Classical Electricity and Magnetism" by Panofsky and Phillips--

   Î¦_m  =  ∫B•ds  =  J / R_m

where R_m  =  âˆ'(l_i /μ_iS_i).  R_m is the magnetic reluctance and J is the "magnetic current."

l_i is the length of an element in the circuit, μ_i is the permeability of that element, and S_i is the cross-sectional area.  In other words, to get the total reluctance in the circuit, you add up the reluctances "R_i" that are connected in series in the circuit.

Since the reluctance of the air gap is very much higher than that of the replacement core material, the circuit flux will increase substantially when you fill the gap.

Ernie Rogers