two equal sections made of steeland iron (relative permeability 1500 and 500, respectively) withcross-sectional area of 1 cm x 1 cm and H = 6 cm, W = 4 cm (seeFigure 1). The iron section was attached to the ceiling while thesteel section was placed on a horizontal support so that 1-mm airgaps were left between the sections. The mass density of steel is7.8 g/cm3. The students consider triggering therelay using a coil (200 turns, resistance 5 Ω) energized by abattery.

What is thetotal reluctance of the steel and iron sections?
What is the magnitude of the magnetic field density that needs tobe established in the gap?
What is the magnitude of the magnetic flux that needs to beestablished in the steel section of the relay?  
What is the required voltage of the battery?

I think i got the first part, but for the next 3parts i am losthelp would be nice..

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