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Physics · 术语表

什么是 Magnetostatic field？

Definition 28.1 University Physics — Year 1 · 第 28 章 — Magnetostatics: Field, Forces, and Dipoles

Steady currents (charges in steady motion) create at every point a magnetic field $\vect B$ (tesla, $\mathrm{T}$ ), defined by the force $q\vect v\wedge\vect B$ it exerts on a moving test charge (Definition 17.1). Its field lines are tangent to $\vect B$ and oriented along it. Permanent magnets create fields of the same nature, from the microscopic currents of their atoms. Orders of magnitude: Earth $5 \times 10^{-5}\,\mathrm{T}$ ; a fridge magnet $0.01\,\mathrm{T}$ ; an MRI scanner $1.5\,\mathrm{T}$ to $3\,\mathrm{T}$ ; the largest steady laboratory fields $45\,\mathrm{T}$ .

Three field maps. A dot is a current toward the reader, a cross a current away. The lines of a wire are circles; those of a loop thread it and close outside; those of a long solenoid are straight and uniform inside and close far away, where the field is weak — in every case they have neither source nor sink.

示例

Example 28.7 (Orders of magnitude)

A $10\,\mathrm{A}$ wire at $1\,\mathrm{cm}$ : $B = 2 \times 10^{-7} \times 10/0.01 = 2 \times 10^{-4}\,\mathrm{T}$ , four times the Earth’s field — the compass does swing. A coil of $100$ turns, radius $5\,\mathrm{cm}$ , $2\,\mathrm{A}$ : $\mu_0NI/2R = 2.5\,\mathrm{mT}$ at its centre. A solenoid of $1000$ turns per metre at $5\,\mathrm{A}$ : $6.3\,\mathrm{mT}$ ; with an iron core of relative permeability $1000$ , several tesla would follow if iron did not saturate near $2\,\mathrm{T}$ — the ceiling of every electromagnet, which is why the strongest fields come from superconducting coils without iron.

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