Electrical / Electronic Theory and Learning Center > Induction

"Core" outside of a coil

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gyula:
Hi ex,

Ok, my assumption of the differing inductances was a possibility,  now I have a go otherwise:  I tend to accept what poynt99 wrote and the reason is that inside the coil if you consider any single turn, the fields of any single turn "sees" the inserted core twice.  How I mean, see this link on a normal solenoid, without core, Fig. 2, explanation is above Fig.2:
http://plasma.kulgun.net/sol_page/

Gyula

exnihiloest:

--- Quote from: poynt99 on 2012-03-05, 13:31:56 ---Ex,

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.

--- End quote ---

This conclusion is not correct. The emf is not proportional to the B field only, the emf is proportional to the flux crossing (wikipedia). Inside the coil, the surface is less than outside, and the B field intensity is more. Outside of the coil, the surface is much larger, B is not constant and we have to integrate B on an infinite surface. Nothing here implies that the flux should be more inside than outside the coil.
It is the same thing with a river: the intensity of the current (= the B field) can be very strong when the water flows in the narrow pipe of a barrage to rotate a generator, but the flow is the same as further downstream when the river is wide (same number of m3/h/by unit of crossed surface)

In my experiment, the ferrite outside of the coil should channelize the outside flux because it represents an easier path for the magnetic flux than air. As it is the same ferrite in both cases when it is inside or outside, we should expect for the same effect because we have the same flux through the same surface.
So the explanation must be more complicated (or more simple  :( ).


poynt99:
Not correct Ex.

This is also from Wicki


--- Quote ---Outside

Magnetic field created by a solenoid (cross-sectional view) described using field lines.

A similar argument can be applied to the loop a to conclude that the field outside the solenoid is radially uniform or constant. This last result, which holds strictly true only near the centre of the solenoid where the field lines are parallel to its length, is important in as much as it shows that the flux density outside is practically zero since the radii of the field outside the solenoid will tend to infinity.

An intuitive argument can also be used to show that the flux density outside the solenoid is actually zero. Magnetic field lines only exist as loops, they cannot diverge from or converge to a point like electric field lines can (see Gauss's law for magnetism). The magnetic field lines follow the longitudinal path of the solenoid inside, so they must go in the opposite direction outside of the solenoid so that the lines can form a loop. However, the volume outside the solenoid is much greater than the volume inside, so the density of magnetic field lines outside is greatly reduced. Now recall that the field outside is constant. In order for the total number of field lines to be conserved, the field outside must go to zero as the solenoid gets longer.
--- End quote ---

exnihiloest:

--- Quote from: gyula on 2012-03-05, 14:10:53 ---Hi ex,

Ok, my assumption of the differing inductances was a possibility,  now I have a go otherwise:  I tend to accept what poynt99 wrote and the reason is that inside the coil if you consider any single turn, the fields of any single turn "sees" the inserted core twice.  How I mean, see this link on a normal solenoid, without core, Fig. 2, explanation is above Fig.2:
http://plasma.kulgun.net/sol_page/

Gyula

--- End quote ---

If you represent the flux with lines, why should it be less lines outside than inside?
Only the density of lines crossing a unit of surface is higher inside than outside, not the number of lines, therefore the flux is the same, it is just crossing a wider surface outside than inside.
But with a ferrite core outside, the ferrite curves the field lines so that they concentrate in the ferrite material, and this is the only interest of my experiment (to try to concentrate the same flux outside than inside. Where is the cause of the failure, I don't yet know):


("The magnetic field is concentrated through the ferrite rod").

poynt99:
And from Hyperphysics:

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