# Homework Help: Inductance of coils

1. Nov 8, 2012

### regisz90

1. The problem statement, all variables and given/known data

There are two coils wined around the same closed iron core. Their inductances on this iron core are measured to be 4 henry, and 9 henry. Then the two coils are connected in series. What will the equivalent inductance be?

2. Relevant equations
...

3. The attempt at a solution
I think that the measured values aren´t the inductances of the coils, but there is also some mutual inductance. The equivalent inductance in series will be L=L1+L2+2M
I just dont know that the 4 Henry and the 9 Henry are in what relation with the L1 and L2 inductances(without mutual)

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 8, 2012

### Staff: Mentor

Hint -- the inductance is proportional to the number of turns __________ (fill in the blank).

3. Nov 8, 2012

squared?

4. Nov 8, 2012

### Staff: Mentor

Yep!

So if you combine two equal coils L1 and L2, you don't get L1+L2, you get ________

And now how can you use this to solve this problem?

5. Nov 11, 2012

### regisz90

its proportional squared, so if the number of turns are N1 and N2, then we get L=k*(N1+N2)^2=k*N1^2 + k*N2^2 + 2*k*N1*N2=L1 + L2 + 2*sqrt(L1*L2).
But I dont know how can i use this to solve the problem. The result is the inductance of the two coils together, and they tried to measure them separately on the same iron core, but i dont know what their result is. L1 and L2? or L1+sqrt(L1*L2) and L2+sqrt(L1*L2)?

6. Nov 11, 2012

### Staff: Mentor

Try making up some numbers for the inductance versus the number of turns squared. The two coils are on the same core, so they will have the same inductance per number of turns squared. Work out the total inductance from there...