Physics
Physics
Problem
A battery is connected to a long solenoid that has an easily removable soft-iron core. A small compass needle can be placed at different points along and around the solenoid. (i) Draw a neat labelled sketch of the magnetic field lines produced in and around the solenoid when the current is switched on and the core is in place. (ii) Predict and explain the behaviour of the compass needle when it is placed (a) at the centre of the solenoid and (b) near one end outside the solenoid. (iii) The core is now slowly withdrawn while the current remains the same. State and explain the changes you would expect in (a) the compass deflections, (b) the spacing of the field lines inside the solenoid, and (c) the strength of the magnetic field outside the solenoid.Intuitive explanation
A solenoid with an iron core is essentially an electromagnet—the iron dramatically amplifies the magnetic field by aligning its own atomic magnets with the coil's field. The key insight is that removing the core weakens the field everywhere because you lose that amplification effect, even though the current stays constant. Watch how the compass responds differently at the center (where field lines are parallel and strong) versus outside the end (where they spread out), and predict how both regions change as iron leaves the picture.
Intuitive explanation
A solenoid with an iron core is essentially an electromagnet—the iron dramatically amplifies the magnetic field by aligning its own atomic magnets with the coil's field. The key insight is that removing the core weakens the field everywhere because you lose that amplification effect, even though the current stays constant. Watch how the compass responds differently at the center (where field lines are parallel and strong) versus outside the end (where they spread out), and predict how both regions change as iron leaves the picture.