Which of the following does NOT determine magnetic field strength?

• A. The ratio of the coil's width to its length
• B. The amount of current flow through the conductor
• C. The number of turns in the coil
• D. The material of the conductor

Answer: (A)

The field strength inside a coil is set by current, how tightly the turns are packed along the axis, and the magnetic permeability of any core. In the standard long solenoid model, B is proportional to the current I and to the turns per unit length n = N/L, with a factor of the core’s permeability: B ≈ μ0 μr n I. The coil’s width (diameter) and its width-to-length ratio do not appear in this fundamental relation, so changing the coil’s diameter while keeping current and turns per length the same does not alter the on-axis field in the ideal case. That geometric aspect, rather than the other factors, is what doesn’t determine the field strength. The current drives the field directly, more turns per length increase it, and using a core with higher permeability concentrates flux to boost the field. The material of the conductor itself generally doesn’t set the field strength in the basic model (though it affects resistance and heating); the essential determinants are current, turns per length, and core properties.