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?
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AThe magnetic field is unaffected; lines remain straight and uniform
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BThe lines of force bend and crowd to pass through the piece of iron (high-permeability path)
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CThe lines of force bend away from the piece of iron, avoiding it
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DAny of the above may occur depending only on the type of iron
Answer
Correct Answer: The lines of force bend and crowd to pass through the piece of iron (high-permeability path)
Explanation
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:
Model the space as a magnetic circuit with parallel paths: air versus iron.Since reluctance R_m ∝ l/(μ A), iron's large μ reduces R_m significantly.Flux chooses the low-reluctance path → field lines concentrate within the iron; external lines bend accordingly.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)