In magnetism, the maxwell is a CGS unit used for measuring which physical quantity?

Introduction / Context:
The study of magnetism uses several different systems of units, including the CGS (centimetre gram second) system and the SI (International System of Units). In SI, the weber is the standard unit of magnetic flux, but in older CGS based descriptions the maxwell is commonly used. Many competitive exams still test whether students can correctly match these traditional units with the physical quantities they represent. This question checks whether you know what quantity is measured in maxwells.

Given Data / Assumptions:

• We are working within the CGS system of electromagnetic units.
• The unit in question is the maxwell.
• We must decide whether it refers to intensity of magnetisation, permeability, magnetic flux or magnetic susceptibility.
• We assume standard textbook definitions of basic magnetic quantities.

Concept / Approach:
Magnetic flux describes the total magnetic field passing through a given area and is analogous to electric flux in electrostatics. In SI units, magnetic flux is measured in webers. In the older CGS system, the corresponding unit of magnetic flux is the maxwell. Intensity of magnetisation (or magnetisation) has the CGS unit emu per cubic centimetre and the SI unit ampere per metre. Permeability has units that relate magnetic flux density to magnetic field strength, and susceptibility is dimensionless. Therefore, the only option that correctly matches the maxwell is magnetic flux.

Step-by-Step Solution:
Step 1: Recall that magnetic flux is the total magnetic field B passing through a surface, often written as phi. Step 2: In SI units, the weber (Wb) is defined as the unit of magnetic flux. Step 3: In the CGS electromagnetic system, the corresponding unit of magnetic flux is called the maxwell. Step 4: Intensity of magnetisation and magnetic susceptibility describe how materials respond to magnetic fields, not total flux. Step 5: Magnetic permeability describes the relation between B and H fields, not the total flux itself. Step 6: Therefore the physical quantity measured in maxwells is magnetic flux.

Verification / Alternative check:
Textbooks on electromagnetism usually include a conversion between units, for example 1 weber = 10^8 maxwells. This conversion clearly shows that both units refer to the same underlying quantity, magnetic flux. Tables of CGS units list maxwell under the heading for flux and gauss as the unit of flux density, further confirming that the maxwell is a unit of flux, not of magnetisation, permeability or susceptibility.

Why Other Options Are Wrong:
Intensity of magnetisation: This quantity measures magnetic moment per unit volume. Its units are not maxwells and it is not a measure of total flux.

Magnetic permeability: Permeability relates magnetic flux density to field strength and has different units, such as henry per metre in SI.

Magnetic susceptibility: Susceptibility is a dimensionless ratio that compares magnetisation to field strength and has no unit like the maxwell.

Common Pitfalls:
A common confusion is between magnetic flux and flux density. Students may mix up the units maxwell (flux) and gauss (flux density in CGS). Another error is to assume that any magnetic quantity must be measured in the same unit as the field itself. To avoid this, remember the parallel between electric and magnetic quantities: charge and flux are totals, while field strength and flux density describe intensity. The maxwell specifically refers to the total amount of magnetic flux passing through an area.

Final Answer:
In the CGS system, the maxwell is the unit of magnetic flux.