Why do 2 solenoids attract each other? why do 2 solenoids, placed on the same axis and carrying current in the same sense, attract each other? I know that they can be modelled as magnetic dipoles, and then the attraction between unlike magnetic poles becomes obvious. But when I try to do it the proper way, using the magnetic field created by one solenoid and fleming's left hand rule to find the direction of force on the other, I am stumped.
Is it not possible to deduce the attraction between them by the direction of the field created and Flemings left hand rule?
 A: "Is it not possible to deduce the attraction between them by the direction of the field created and Flemings left hand rule?"
Yes, it is possible, but you have to remember that outside the solenoids their fields are not axial, but splay out, and so have a component radial to the axis. Use the left hand rule with first finger pointing radially out from the axis (or radially towards the axis) and the second finger pointing tangentially, that is in the current direction for any part of a turn of the 'second' solenoid.
A: You can approximate a solenoid as a series of current loops. When we look at two loops of opposing current it looks as follows

The black lines are current loops. Red indicates magnetic field and blue indicates force. We determine the effect of the left loop's magnetic field on the right loop. Using the right hand rule$^\dagger$ we can get the direction of the magnetic field (little red circle). This gives us the direction of the magnetic field on the right loop. Using another right hand rule we can determine the direction of force. On the right hand side of the right loop we have the same force but in the opposite direction. Because it's farther away the force is weaker and the net force is attractive. We would have to sum over all the little forces in the loop to get an exact answer.
It sounds like you were imagining both loops of current to be rotating in the same way so both loops would be copies of each other. This can't be right: if you were to replace two identical loops with bar magnets they would be pointing in the same direction. Since north-north and south-south matches up they would be repelling.
$\dagger$ What is this black magic called a left hand rule??
