Deriving Ampere's law from the Biot-Savart law

1. Sep 11, 2014

fayled

If we write the Biot Savart law as
B(r)=μ0/4π∫(J(r')xn/n2)dV'
where B is the magnetic field which depends on r=(x,y,z), a fixed point, J is the volume current density depending on r'=(x',y',z'), and n=r-r', a vector from the volume element dV' at r' to the point r. Note we integrate over the primed coordinates as J, the source of the current varies with these.

Then take the curl, making use of curl(AxB)=A(.B)+(B.)A-(A.)B-B(.A), and noting .J=0 (it depends on the primed coordinates) and that (n/n2.)J=0 for the same reason, we get
xB0/4π∫J(.n/n2)dV'-μ0/4π∫(J.)n/n2dV'.

Now term two can be shown to integrate to zero, which I understand (incidentally, the book says this second term is integrated over a volume enclosing all current, as I suspected - this is related to my problem below), but I have a problem with term 1. We get
xB0/4π∫J(.n/n2)dV'
and .n/n2=4πδ3(r-r'). So
xB0/4π∫4πJ(r'3(r-r')dV'
xB0J(r'3(r-r')dV'
Now according to my book, this reduces to
xB0J(r), which makes sense in a way because we can say
xB0J(r3(r-r')dV'
because only the value of J at the 'spike' is actually useful anyway. This is constant so
xB0J(r)∫δ3(r-r')dV' and then this integral is simply one giving the desired result.

However, we're integrating over (x',y',z') and so varying r', keeping r fixed. As our integral only need cover all of our current, and r could be outside of our current distribution, (according to my brain) we need not even integrate over the 'spike' of the delta function at r'=r, which makes the integral zero.

What is it I am misunderstanding? Thanks.

Edit: If the answer is something along the lines of 'we may aswell integrate over all space because there's no current anywhere else anyway', I ask, if there's no current anywhere else anyway, why do we get two different answers by integrating/not integrating over all space.

Last edited: Sep 11, 2014
2. Sep 11, 2014

jambaugh

You are not wrong!

You are not wrong... just considering a special case. IF you pick a point outside the current distribution the curl of B is zero as your mathematical intuition indicates. You are deriving Ampere's law -after all- which states that the curl of B at a point is proportional to the current density at that point. But r need not be outside the current distribution in which case you DO need to integrate over the spike and wherein B and J are not zero but still proportional.

All is right with the world.

3. Sep 11, 2014

fayled

Oh dear, that is pretty obvious now Thankyou :)

4. Sep 12, 2014

vanhees71

Well, and if $\vec{r}$ is at a point outside the region where an electric current is flowing, your equation is correct too (for stationary currents only, of course!), because there $\vec{j}(\vec{r})=0$. The local Maxwell Equations are always right!