U-shaped iron core with I-shaped iron core vs without

fojkehelky

I have two coils standing next to each other. One is connected to a power supply and the other one is connected to a lamp.

When I put an I-shaped iron core through both coils the lamp shines brighter than without the iron core. The same thing happens when I use a U-shaped iron core. When I put an I-shaped iron core on top of the U-shaped iron core the lamp shines much brighter.

I think I understand why an iron core makes the lamp shine brighter. What I don't understand is why the lamp shines much brighter when I use a U-shaped iron core with an I-shaped iron core on top of it. What is the difference between on the one hand having only an I-shaped iron core or only a U-shaped iron core and on the other hand having a U-shaped iron core with an I-shaped iron core on top of it?

I have been looking for an explanation but I can't find anything. Grateful for help.

berkeman

What you are doing is making transformers where you couple the changing magnetic field from one coil to the other one (primary to secondary). When you have a "closed" magnetic path (when you have the iron path a closed circuit), it is a much better transformer. The more the magnetic field has to travel through air to form a closed path, the worse the transformer action is.

http://en.wikipedia.org/wiki/Transformer

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fojkehelky

thanks for the reply

So does the magnetic field always form a closed path? This makes me think: is there a small part of the magnetic field that travels through air to form a closed path even when I have a square-shaped iron core=a closed magnetic path (U-shaped+I-shaped)?

berkeman

Yes, very good observation on your part. Even when the iron pieces are in physical contact, there is still a "gap". In transformer design, we sometimes put an explicit gap in the magnetic path, for various reasons. Usually you want to minimize the gap, and we even go so far as to mirror polish the mating surfaces to minimize the gap.

To have no gap at all in the magnetic path, you need to use a toroidal core. But it is harder (and more expensive) to wind your coils on a toroid, so toroidal transformers are only used in special applications (like where you want to minimize the physical size of the transformer). Most transformers that use a bobbin to hold the coils will need to be made up of 2 or more core pieces that fit together with the bobbin placed onto them.