# Homework Help: Self inductance question

1. Aug 28, 2009

### nabliat

http://i26.tinypic.com/250tkyh.jpg

the formula for magnetic field
is like they show but its not divided by l
like they did

i cant see a reason why they present it that way
??

2. Aug 28, 2009

### gabbagabbahey

You mean in the equation for $M$?

If so, $M$ is the mutual inductance and by definition,

$$\Phi_{\text{total}}=MI\implies M=\frac{\Phi_{\text{total}}}{I}$$

3. Aug 28, 2009

### nabliat

no
when they found the magnetic field
and the expression has l in the denominator

why??

4. Aug 28, 2009

### gabbagabbahey

Sorry, the lower case 'L' in your post looked a lot like an upper case 'i' the first time i read it....

$$\frac{N_b}{l}$$ would give you th number of turns per unit length in the solenoid, which is what $n$ represents in the version you are probably used to seeing: $B=\mu_0 n I$.

5. Aug 28, 2009

thanks :)