If the current in a long straight wire doubles, what happens to the magnetic field at a fixed distance?

If the current in a long straight wire doubles, what happens to the magnetic field at a fixed distance?

• A. It quadruples
• B. It doubles
• C. It remains the same
• D. It halves

Answer: (B)

The magnetic field around a long straight wire grows directly with the current. At a fixed distance from the wire, the distance is constant, so the field strength scales with the current: B = μ0 I /(2π r). Since μ0 and r are fixed, doubling the current makes the magnetic field magnitude double as well.

The magnetic field around a long straight wire grows directly with the current. At a fixed distance from the wire, the distance is constant, so the field strength scales with the current: B = μ0 I /(2π r). Since μ0 and r are fixed, doubling the current makes the magnetic field magnitude double as well.