Magnetic fields of a straight wire and circular loop
Field geometry follows the current distribution.
Key knowledge
An ideal infinitely long thin straight wire carrying steady current has circulating field magnitude B=μ₀|I|/(2πr). A thin circular loop of radius R has axial field B_z=μ₀IR²/[2(R²+z²)^(3/2)], where positive I follows the right-hand orientation about +z. The loop centre has B_z=μ₀I/(2R); far along the axis the field falls as distance cubed.
Values and equations
B_wire=μ₀|I|/(2πr); B_z=μ₀IR²/[2(R²+z²)^(3/2)]
- B_wire,B_z
- Straight-wire field magnitude and signed loop axial field T
- μ₀
- Vacuum permeability H m⁻¹
- I
- Steady oriented current A
- r,R,z
- Wire distance, loop radius and axial position m
Conditions and scope
- Vacuum magnetostatics, r>0 outside the thin wire and R>0; the straight-wire result is an infinite/long-wire idealization with distant return path.
- Nearby return wires, finite-length effects, material response and changing currents modify the field. μ₀ is used symbolically and is not assigned the pre-2019 exact numerical value from the textbook.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Samuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)2 sources
- University Physics Volume 2, §12.2: magnetic field due to a thin straight wireSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
- University Physics Volume 2, §12.4: magnetic field of a current loopSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
Evidence and review
Read §12.2 long-wire Biot–Savart integral/result12.5 and §12.4 on-axis loop integral12.7. Axes are relabelled consistently. Exclude outdated exact μ₀ numerical claims and do not equate the full loop field with its far-dipole approximation.
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