A solenoid is constructed by winding an insulated copper on a long test tube. A constant direct current is…

Analyze the question: We want the material that will not strengthen the magnetic field of a current-carrying solenoid when inserted into it.

Concept: The magnetic field inside a solenoid is B = μnI, where μ = μ₀μᵣ.

• n = turns per unit length
• I = current
• μᵣ = relative permeability of the core material

A core with a high relative permeability (μᵣ ≫ 1) greatly increases B. These are ferromagnetic materials: iron, steel, nickel and cobalt. The domains in these materials line up with the solenoid's field and add to it.

A non-magnetic material such as brass (an alloy of copper and zinc) has μᵣ ≈ 1. It behaves almost like air, so it adds essentially nothing to the field.

Checking each option:

• A. Steel rod: ferromagnetic, so the field increases (steel keeps its magnetism well).
• B. Nickel rod: ferromagnetic, so the field increases.
• C. Brass rod: non-magnetic, μᵣ ≈ 1, so there is no significant increase.
• D. Soft iron rod: very high permeability, so it gives the biggest increase. This is why it is used in electromagnets.

Answer: The brass rod will not increase the strength of the field, so the answer is C.

Exam tip: Remember the ferromagnetic materials as "Iron, Cobalt, Nickel, Steel". Brass, copper, aluminium, wood and plastic are not good core materials. When a question asks for what will not increase the field, look for the non-magnetic material.