# Magnetic field due to a solenoid.

1. Nov 1, 2013

### PhysicoRaj

Hi.
I have a solenoid, of length l, radius r, which has n turns of copper wire wound on it, carries a current of I amperes, and has a ferromagnetic core of relative permeability μr. How do I determine the magnetic field at a point in free space, x meters from the center of the solenoid?
Also, how to calculate the force of attraction experienced by a ferromagnetic substance(whose magnetic properties are known) due to the same solenoid, placed at that point?

Last edited: Nov 1, 2013
2. Nov 1, 2013

3. Nov 1, 2013

### PhysicoRaj

Thanks.
The problem I came upon was that the field inside the solenoid will be influenced by the permeability of the core whereas the point I wish to measure the field is in a medium of different permeability.
I used B=μnI/l which gives the field inside the solenoid. For the field outside it, what expression should I use so as to switch over to the permeability of free space?

Can I use the below equation(total field by integrating the equation for a current loop) by just replacing μ° by μ°μr?
B=(μ°/4∏)[nIr2/2(r2+x2)3/2]
Wouldn't this mean the point is also in the same medium as the core?

Last edited: Nov 1, 2013
4. Nov 1, 2013

### Simon Bridge

That's pretty much it yeah - remembering how the feld changes at the boundary ... you basically have materials with different refractive indexes.

5. Nov 1, 2013

### PhysicoRaj

Thanks a lot!