Magnetic fields and ferromagnetic materials – effect of inserting iron When an unmagnetized piece of iron is placed in a magnetic field, what happens to the magnetic lines of force in the surrounding space?

Introduction / Context:
Field line sketches are a useful qualitative tool in magnetostatics. Ferromagnetic materials like iron have very high relative permeability, so they substantially alter nearby magnetic fields by providing a low-reluctance path for magnetic flux. This principle underlies transformer cores, inductors, and magnetic shields.

Given Data / Assumptions:

• Piece of iron is initially unmagnetized but magnetically soft.
• External magnetic field is applied (from a permanent magnet or current-carrying coil).
• Quasi-static conditions: no rapid transients or eddy current effects considered.

Concept / Approach:

The magnetostatic field tends to concentrate in regions of higher permeability because magnetic reluctance is lower. Iron, with large μ_r, channels magnetic flux, causing lines of force to bend toward and pass through it. This increases flux density inside the iron while reducing it in surrounding air gaps, a fact exploited to focus flux in magnetic circuits.

Step-by-Step Solution:

Verification / Alternative check:

Finite-element simulations and iron filings experiments clearly show flux concentration in and around iron shapes placed near magnets or coils.

Why Other Options Are Wrong:

(a) ignores permeability contrast. (c) would apply to diamagnetic or strong superconducting expulsion, not ordinary iron. (d) iron types vary in μ and hysteresis, but the general effect is flux attraction and concentration.

Common Pitfalls:

Assuming field lines are physical objects; they are visualization aids. Also, neglecting saturation: at very high flux densities, iron may saturate and the concentrating effect diminishes.

Final Answer:

The lines of force bend and crowd to pass through the piece of iron (high-permeability path)