Electrical / Electronic Theory and Learning Center > Induction

"Core" outside of a coil

(1/7) > >>

Why a coil has a different behavior when a magnetic core is place inside and when it is placed outside? By "outside core", I mean a hollow magnetic cylinder in which a coreless coil is placed (see attachment).

I led the following experiment:
- A LC circuit is tuned to a resonant frequency of 600 Khz. The coil has a large diameter in order to place a ferrite cylinder inside. This cylinder acts as a ferrite rod. When I put this ferrite core in the coil, the resonant frequency is lowered down to 378 Khz, which means a variation of -37% of the frequency.
- Another LC circuit is also tuned to a resonant frequency of 600 Khz. The coil is of smaller diameter so that it can take place inside the same ferrite cylinder. When I put the cylinder around the coil, the resonant frequency is lowered down to 557 Khz, which means a variation of only -7% of the frequency.

In both cases, the field lines can pass through the ferrite, either through the "inside core" for the first case or the "outside core" in the second case. The magnetic circuit in which the field lines are looped is constituted in both cases of a half pass in air and a half pass in ferrite. There is the same flux inside and outside of the coil because the flux is supposed to be conservative. So why the ferrite effect in the second case is less than in the first one?

Hi exnihiloest,

I assume your smaller diameter coil has got less self inductance than the large one.  This means the small coil needs a higher value capacitor to get resonance at 600kHz than the large coil does.
Now the detuning effect of the cylinder core for the small coil can only be less than for the large coil from the resonant frequency point of view because a smaller inductance is able to change its L value in a smaller percentage than the large one.  The L/C ratio is smaller for the small coil case, hence the influence of the core on the L is also less.


Thanks for the reply, Gyula.
I had chosen coils of similar inductance. For the same value of capacity, the small coil oscillates on 840 Khz and the larger on 600 Khz.

Nevertheless, well done: I had to adjust the capacity as you said (I use a variable capacitor), and this can be the cause of a biased experiment.
So to take into account your relevant remark, here is the result of the same experiment with the small coil, but by keeping the same capacity as for the larger one:
- without core: 843 Khz
- with the "outside core": 780 Khz
The variation is: 7,47%.
Therefore the weakness of the inductance change can't be explained this way although we see you are right when you say that the detuning is less when the capacity is more.

The mystery remains.  :)
I have noticed for a long time this phenomenon of the weak effect of the presence of an external magnetic material, but it appears to me abnormal only today, after thoughts about the "Non-conservative Fields Experiment" seen in the other thread.


It's the same phenomenon with the Lewin experiment; the B field intensity is much larger within the diameter of the coil, than outside of it.

The electric field set up from the changing B field INSIDE the coil is co-axial with the coil, therefore it is in the perfect position to induce an emf in the coil.

The less intense B field returning OUTSIDE the coil is not co-axial with the coil, therefore the weaker electric fields produced outside the coil are not able to induce as much emf, if any at all.


Poynt beats me into it.  The field lines is much more dense within the coil than outside the coil. 


[0] Message Index

[#] Next page

Go to full version