Solenoid Circuit: Understanding Needle Direction with Open and Closed Switch

[joedirt]

Homework Statement

A soft iron bar in a solenoid is placed in a circuit. There are two compasses, 1 is on the West side of the iron bar and the other is to the North. The circuit switch is open, which way will be compass needles be pointing?
Which way will the compass needles be pointing if the switch is then closed?


Is there a rule to solenoids?

 

[rl.bhat]

when the switch is open, both needles will point towards north, because soft iron bar is not magnetized.
When the switch is closed, soft iron gets magnetized. Then the the needle placed west side will point either east or west depending on the direction of the current in the circuit.
If you follow the lines of magnetic field, you can see that the needle on the north side will be in the opposite direction compared to the needle on the west side.