Permeability is defined as what?

Permeability is a fundamental property of materials that indicates how easily a magnetic field can penetrate and affect them. It is specifically defined as the ratio of magnetic flux density (the amount of magnetic field lines passing through a unit area) in a substance to the magnetizing force (the external magnetic field applied) that produces it. This relationship highlights how responsive a material is to magnetization when subjected to an external magnetic field.

In practical terms, a material with high permeability will allow magnetic field lines to pass through it more easily, making it suitable for applications in electromagnetism, such as transformers and inductors. This concept is key in understanding how different materials react under magnetic influences and is critical for designing electromagnetic devices.

The other choices describe different physical properties but do not align with the definition of permeability. One option refers to electrical conductivity, which is unrelated to magnetic properties, while another discusses signal speed in a medium, which pertains to electromagnetism but focuses on wave propagation rather than the magnetic properties of materials. The last option mentions the magnetism a material can hold, which touches on magnetic saturation but does not accurately define permeability itself.