ampere per metre

SI coherent derived unit

Name	Symbol	Quantity	Base units
ampere per metre	m⁻¹ A	magnetic field strength	m⁻¹ A
The ampere per metre, symbol m⁻¹ A, is the SI coherent derived unit of magnetic field strength.
Definition			c^-1 Δν_Cs² e
$1 \mspace{4mu} \text{m}^{-1} \mspace{4mu} \text{A} \mspace{6mu} = \dfrac{299 \mspace{4mu} 792 \mspace{4mu} 458}{(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^2(1.602 \mspace{4mu} 176 \mspace{4mu} 634 \times 10^{-19})} \mspace{6mu} \dfrac{{\Delta \nu _{Cs}}^2 \mspace{4mu} e}{c}\\ \\ \\ 1 \mspace{4mu} \text{m}^{-1} \mspace{4mu} \text{A} \mspace{6mu} \approx 2.214 \mspace{4mu} 270 \mspace{4mu} 027 \mspace{4mu} 258 \mspace{4mu} 733 \mspace{4mu} 941 \times10^{7} \mspace{6mu} c^{-1} \mspace{4mu} {\Delta \nu _{Cs}}^2 \mspace{4mu} e$

Magnetic fields

A magnetic field is a vector field that describes the magnetic influence of electric currents and magnetised materials. Magnetic fields are created by magnetised material and by moving electric charges (electric currents) such as those used in electromagnets.

When an electric current flows through a straight conductor, the strength, H, of the magnetic field it creates is directly proportional to the size of the current, I, and inversely proportional to the radial distance, r from the conductor.

Using SI coherent units,

$\mathbf{H} = \dfrac{I}{2 \mspace{2mu} \pi \mspace{2mu} r}$

where:

H is the strength of the magnetic field in amperes per metre, symbol A m^-1,
I is the electric current in amperes, symbol A,
r is the radial distance from the conductor in metres, symbol m.

When an electric current flows through a coiled conductor, the strength of the magnetic field, H, inside the coil is directly proportional to the size of the current, I, and the number of coils of the conductor, N, and inversely proportional to the length of the coil, L.

Using SI coherent units,

$\mathbf{H} = \dfrac{I \mspace{2mu} N}{L}$

where:

H is the strength of the magnetic field in amperes per metre, symbol A m^-1,
I is the electric current in amperes, symbol A,
N is the number of coils of the conductor,
L is the length of the coil in metres, symbol m.

Magnetic flux density

When a magnetic field is applied to a material, the resulting magnetic flux density, B, is directly proportional to the strength of the applied magnetic field, H, and the magnetic permeability, μ, of the material.

Using SI coherent units,

$\mathbf{B} = \mu \mspace{2mu} \mathbf{H}$

where:

B is the magnetic flux density in teslas, symbol T, (or webers per square metre, symbol Wb m^-2),
H is the strength of the applied magnetic field in amperes per metre, symbol A m^-1,
μ is the magnetic permeability in henries per metre, symbol H m^-1.