Decrease in flux density (Ruse Trial Q) (1 Viewer)

[superstar12]

From 2011 James Ruse Trial:



Weights are added to the iron bar until the bar pulls away from the core. Explain why the brightness of the lamp momentarily increases at the instant the rod stops touching the iron core. (2 marks)

The sample answer is: "The flux density in the coil decreases, inducing an emf (Faraday’s law) and then an induced current in the coil which opposes the change (Lenz’s law)."

Could someone please explain why flux density in the coil decreases?

 

[iforgotmyname]

I think its something along the lines of mutual induction.

The magnetic field produced by the electromagnet induces eddy currents into the iron bar which has its own magnetic field which effects the coil

When the iron bar is pulled away, there would be a change in magnetic flux (less flux in electromagnet cause it gets pulled away) so emf will be induced into the magnetic bar to pull it closer to electromagnetic to minimize flux change according to lenz's law.