dimivat said:

For the question that asks the distance between the rod, if the mass they gives you decreases with the increase in current doesn't that mean the distance is changing as a result of the second rod being atracted to the first?

What I did:

the reading on the scale, R, reads Rg as resultant force. ie

Weight pulling rod down - Force pulling rod up = Rg

mg - F = Rg

elementary maths tells us :

-ΔF = ΔRg

but F = kLI1I2/d

the only change is in current, (distance doesnt change)

making:

-ΚLI1(ΔI2/ΔR)/g = d

ΔR/ΔI2 is of cause the gradient of the graph.

so you just sub it all in..

there we go, sorry, but i am 4U math, i prefer to derive everything.