# The logic of the self inductance formula

1. Aug 28, 2009

### nabliat

$$M_{12}=\frac{N_2\phi _{12}}{I_1}$$

i need to remmeber it

but i cant see the logic of
the formula
??

2. Aug 30, 2009

### roam

I thought the formula looked something like: $$\frac{\mu_{0}N^2 A}{l}$$. A is the cross section & magnetic field inside the solenoid is $$B=\mu_0 NI/l$$, flux through one loop is $$BA=\mu_0 N IA/l$$ and $$\Phi= \mu_0 N^2 I A/l$$ is the total flux. The inductance is just total flux divided by current.