# Ampere's law

1. Apr 12, 2014

### Mr-R

Dear all,

I am trying use Ampere's law and came across a problem which is a result of my lack of understanding.

$\oint$ B.dl=uI
My lecturer derived the above equation by defining a point in the middle of a potato shaped loop. Where he stated that dl in this case is just rdθ. (r is the line joining dl to the point defined in the middle). And using B=$\frac{uI}{2πr}$ he integrated it with respect to rdθ from θ=0 to 2π to get $\oint$ B.dl=uI.

Now, I am trying to do the same for a circular loop. Obviously I got the same answer uI. But its wrong as it should be $\frac{uI}{2r}$. The problem is that I am using the same approch he used for the general shape but not getting the same answer. I am using B=$\frac{uI}{2πr}$ where I should actually use B=$\frac{uI}{4πr^{2}}$

Could someone explain why can't I do that?

P.s sorry if this is messy. It is my first time.
Rashid

2. Apr 12, 2014

### Mr-R

I think I got it guys. Mods can close this.