# If we know the inductance of an iron-cored coil to be 200mH, and we

1. Sep 28, 2011

### dimpledur

If we know the inductance of an iron-cored coil to be 200mH, and we remove the iron core such that the core is now air, how exactly can we determine what the new inductance is with just this information?
I played around with the equation for inductance, however, I could not seem to get it absent of the permeability for the iron core, which is not provided. Any suggestions are greatly appreciated.

2. Sep 28, 2011

### yungman

Re: Inductance

Because you can't get away without the $\mu\;$ of the iron core!!!! Look up the composition and look up the $\mu\;$.

3. Sep 28, 2011

### sandy.bridge

Re: Inductance

How do you "look" up the permeability? Permeability isn't a constant, in fact it varies according to that cores specific B-H graph. Also, the question only gives the value I=200mH, however, it is able to be solved (considering the question explicitly points out "you most certainly do have enough information to answer this.)

4. Sep 28, 2011

### es1

Re: Inductance

This sounds like homework to me. Are you sure the instructor did not provide a typical number for the permeability of an iron core during lecture? If not, google it and use that.

The question asks for 'how' so as long as you show your technique you should be fine... :)

5. Sep 29, 2011

### jim hardy

Re: Inductance

look carefully into "permeability"

and what is inductance...

there's u and u-sub0

one is permeability of free space

other is ratio of permeability of your core material (iron?) to that of free space
(iron's is higher)

you have replaced an iron core with air

so inductance decreased

and it's readily calculable by the formulas

but beware - in real world nearby iron will affect your result.
for example do not attempt to measure inductance of a long solenoid that's lying on a concrete floor, for the rebar in floor affects your readings..