Hi everyone, long time lurker, first time poster. Can someone please clear this up for me? Two wires running parallel to each other. I think its just an algebraic addition of the usual

B(r)=u_0i/2pir

but instead of r, we are using D and d

aslso i apologise for my formulae, if someone could point me to the tex thread ill redo them

## Homework Statement

By summing the magnetic field due to currents of magnitude i flowing in both the wires in the senses you found in part (c), show that, on the line joining the two wires, the magnetic field strength is given by =

`

B(r)=u_0i/2pir

## The Attempt at a Solution

B=(u_0i/2piD)+(u_0i/2pid)

which i get to be

B=(u_0i)/pi(D^2-d^2)

the question says it should be

B=(u_0iD)/pi(D^2-d^2)

Where is that extra D on the numerator from?

Thanks