What determines the strength of a magnetic field?

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

Answer: (A)

Magnetic field strength is set by how much electric current is pushed through a conductor. The field around a straight wire grows in direct proportion to the current: doubling the current doubles the field at the same distance. In a coil or solenoid, the field also depends on turns and geometry, since it scales with current times the number of turns per length, but the fundamental driver you control to change the field is the current itself. The width-to-length ratio doesn’t set the field strength by itself, the material mainly affects resistance, and while adding more turns can boost a coil’s field, that effect works through current as well.