Magnetism and Electromagnetism
A certain amount of current flows through a ring shaped coil with fixed number of turns. How does the magnetic induction B varies inside the coil if an iron core is threaded into coil without dimensional change of coil?

Decreases
Remains same
Increases
First increases and then decreases depending on the depth of iron in¬sertion

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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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