Magnetic field terminology in electromagnetism: A collection of magnetic lines of force that extend externally from the north pole to the south pole of a magnet is called the magnetic __________.

Introduction / Context:
In magnetic circuits and electromagnetic devices, we visualize fields using lines of force. Knowing the correct nomenclature is essential for analyzing transformers, motors, sensors, and magnetic materials in electronics and power engineering.

Given Data / Assumptions:

• A permanent magnet or an energized electromagnet is present.
• We refer to the external field that emerges from the north pole and re-enters at the south pole.
• Standard magnetic circuit terminology is used.

Concept / Approach:
The total quantity of magnetic field lines (conceptually) in a region is called magnetic flux, typically symbolized by phi and measured in webers. Flux density B quantifies flux per unit area and is measured in tesla. Other terms like reluctance, hysteresis, and retentivity describe different magnetic properties, not the lines themselves.

Step-by-Step Solution:

Verification / Alternative check:
Textbooks and standards define flux (weber) as the measure of the magnetic field’s presence, while flux density B (tesla) is flux per area. This aligns with the description of lines of force.

Why Other Options Are Wrong:

• Reluctance: Magnetic “resistance” of a path, analogous to electrical resistance.
• Retentivity: Ability to retain magnetization (residual magnetism).
• Hysteresis: The lagging effect of magnetization relative to the applied magnetizing force, producing the hysteresis loop.

Common Pitfalls:
Mixing flux (quantity of field) with flux density (field strength per area) or confusing material properties with field descriptions.

Final Answer:
flux