so this is part 2 of my original question

Find the electric ﬁeld on the axis at 3.5 m
assuming that the ring is a point charge at
the origin.

Question

so this is part 2 of my original question

Find the electric ﬁeld on the axis at 3.5 m
assuming that the ring is a point charge at
the origin.

jamesj · Answer

Hence what is r in this case?

anonymous · Answer

if I do the same pythagoras theorem then I get 3.5m

jamesj · Answer

Right, but the question says: "Forget Pythagorus; consider the ring to be just a point at the origin", which means it is 3.5 m away.

anonymous · Answer

soo...

jamesj · Answer

Then the electric field is of magnitude
$$ E = \frac{kq}{r^2} $$ just as before. 

It is important that you learn this formula.  It's important and it comes from the Coulomb law formula as I showed you before.