# Electrical length of wire wound on magnetic core?

1. Jan 2, 2016

### BHY-BK

Hi everyone,

I read somewhere that when a coil is wound on a magnetic core it's electrical length becomes longer than it's physical length. I remember seeing this formula

L=l * sqrt Ue

Where L= Electrical length
l=actual (physical) length
Ue=Core permeability

Does anyone know if this is correct and if so what are the units used (Feet, meters)? I tried searching for it again but could not find it.

2. Jan 2, 2016

### davenn

I cannot find any references to this ....

or this ....

you need to supply some sources

Dave

3. Jan 2, 2016

### Staff: Mentor

It sounds like you are referring to the change in inductance when a wire is wound on a ferrite core versus an air core, or even compared to the inductance of the same length of a straight wire or a single turn.

Like Dave, I'm not familiar with the equation you list. Are you just wanting to compare ferrite-based and air-core inductances?

4. Jan 2, 2016

### Averagesupernova

Something similar to this happens in helical resonators. A coil of wire open at one end and grounded at the other is encased in a shielded enclosure. There are coupling loops adjacent to the resonating coil for coupling power into and out of the filter. The resonant frequency is typically much lower than the 1/4 wave length of the wound coil. This is a very brief description of what happens. There is quite a bit more to it.

5. Jan 2, 2016

### BHY-BK

I had it written down but can't find out where I originally got it from?
It doesn't make sense to me as I don't understand what effect the core permeability would have on the length?

6. Jan 2, 2016

### Staff: Mentor

Not "length", "equivalent length for the same inductance". Are you familiar with that concept yet?

7. Jan 2, 2016

### BHY-BK

Are you saying you can have lines with the same electrical length but different inductances, I do understand that.

8. Jan 2, 2016

### Averagesupernova

If you take a 1/2 wave length of wire that resonates at some frequency and then insert an air core inductor in series with it, it will resonate at a lower frequency. If you then pull this coil apart so the whole assembly becomes a straight wire again it will NOT resonate at the lower frequency, it will likely resonate at some place in between the original and the lower. In other words, we are not just spooling away extra wire. It does not matter if there is a core or not, the effect of inductance is the same. That is not to say the core makes no difference at all. Is this what you are referring to?

9. Jan 2, 2016

### Staff: Mentor

Same *physical* length, with different "electrical lengths".