Magnetism and Electromagnetism
For a given wire-wound core, an increase in current through the coil

Decreases the flux density
Reverses the flux lines
Increases the flux density
Causes no change in flux density

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Magnetism and Electromagnetism
Main causes of noisy solenoid are

strong tendency of fan out of laminations at the end caused by repulsion among magnetic lines of force
Both of the listed here
uneven bearing surface, caused by dirt or uneven wear between moving and stationary parts
None of these

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