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Physics · Glossary

What is Magnetostatic field?

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

Steady currents (charges in steady motion) create at every point a magnetic field B→\vect B (tesla, T\mathrm{T}), defined by the force qv→∧B→q\vect v\wedge\vect B it exerts on a moving test charge (Definition 17.1). Its field lines are tangent to B→\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 5×10−5 T5 \times 10^{-5}\,\mathrm{T}; a fridge magnet 0.01 T0.01\,\mathrm{T}; an MRI scanner 1.5 T1.5\,\mathrm{T} to 3 T3\,\mathrm{T}; the largest steady laboratory fields 45 T45\,\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.
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.

Examples

Example 28.7 (Orders of magnitude)

A 10 A10\,\mathrm{A} wire at 1 cm1\,\mathrm{cm}: B=2×10−7×10/0.01=2×10−4 TB = 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 100100 turns, radius 5 cm5\,\mathrm{cm}, 2 A2\,\mathrm{A}: μ0NI/2R=2.5 mT\mu_0NI/2R = 2.5\,\mathrm{mT} at its centre. A solenoid of 10001000 turns per metre at 5 A5\,\mathrm{A}: 6.3 mT6.3\,\mathrm{mT}; with an iron core of relative permeability 10001000, several tesla would follow if iron did not saturate near 2 T2\,\mathrm{T} — the ceiling of every electromagnet, which is why the strongest fields come from superconducting coils without iron.

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