Wirewound core behavior: For a given magnetic core with a coil, what happens to flux density when the coil current is increased?

Introduction / Context:
Coil current and magnetic flux density are closely linked through magnetomotive force and the material’s magnetic response. This question tests understanding of how increasing current influences flux in a core before saturation effects dominate.

Given Data / Assumptions:

• Fixed core geometry and material.
• Increased current through the coil turns.
• Operation not deep into saturation (linear region assumption).

Concept / Approach:
Magnetomotive force (mmf) is F = N * I, measured in ampere-turns. Magnetic field strength H in a core of mean length l is H = F / l = N * I / l. Flux density B is B = μ * H, where μ is the core’s permeability. Thus, as I increases, H increases, and therefore B increases proportionally in the linear region.

Step-by-Step Solution:

Verification / Alternative check:
Using a B-H curve of the core material, small increases in H (from more current) lead to increases in B until approaching saturation, where the slope reduces. Measurements with a Hall probe or fluxmeter corroborate this in lab experiments.

Why Other Options Are Wrong:

• Causes no change: Contradicts B = μ * H and H ∝ I.
• Reverses flux lines: Reversal requires reversing current direction, not merely increasing magnitude.
• Decreases flux density: Would require reduced current or reduced permeability.

Common Pitfalls:

• Ignoring core saturation, which can limit further B increase at very high I but does not cause a decrease by itself.
• Confusing flux direction (set by current direction) with magnitude (set by current size).

Final Answer:
increases the flux density