I'm having a hard time trying to understand a very basic electromagnetism related question, related to how a solenoid valve works. Consider the image below:
Why is there a force exerted on a ferromagnetic material (at rest, i.e. zero initial velocity) concentric with a solenoid fed with DC current?
I would say that besides the on/off moments, where there is a variation of the magnetic field, the field will be constant and therefore no force would exist.
What am I missing here?