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Magnetic field density

Magnetic field density, otherwise known as magnetic flux density, is essentially what the layman knows as a magnetic field - akin to a gravitational or electric field. The SI unit of magnetic flux density is the tesla. 1 tesla = 1 weber (unit) / square metre.

It can be more easily explained if one work backwards from the equation: B=\frac {F} {I l} \,


B is the magnitude of flux density in teslas

F is the force in newtons experienced by a wire carrying

I amperes of current

l metres in length

Demonstration of the left hand rule
Demonstration of the left hand rule

So, one can see for a magnetic flux density to equal 1 tesla, a force of 1 newton must act on a wire of length 1 metre carrying 1 ampere of current.

1 Newton is a lot of force, and is not easily accomplished. To put it in perspective: the most powerful superconducting electromagnets in the world have flux densities of 'only' 20T.

This is true obviously for both electromagnets and natural magnets, but a magnetic field can only act on moving charge - hence the current, I, in the equation.

Indeed, the equation can be played around with to incorporate moving single charges, ie protons, electrons, and so on via F = BQv


Q is 1 coulomb of charge

v is the velocity of that charge in metre per second

Fleming's left hand rule can be used to determine the direction of motion/current/polarity from any two of those, as seen in the example.

Other units of magnetic flux density are

SI units

SI electromagnetism units

Name Symbol Dimensions Quantity
ampere (SI base unit) A C / s Current
coulomb C A · s Electric charge, Quantity of electricity
volt V J / C = kg m2 / (s2 C) Potential difference
ohm Ω V / A = kg m2 / (C2) Resistance, Impedance, Reactance
ohm &middot metre Ω &middot m kg m3 / (C2) Resistivity
farad F C / V = C2 s / (kg m2) Capacitance
farad per metre F / m C2 s / (kg m3) Permittivity
reciprocal farad F−1 kg m2 / (C2 s) Elastance
siemens S Ω−1 = C2 / (kg m2) Conductance, Admittance, Susceptance
siemens per metre S / m C2 / (kg m3) Conductivity
weber Wb V &middot s = kg m2 / (C s) Magnetic flux
tesla T Wb / m2 = kg / (C s) Magnetic flux density
ampere / metre A / m C / (s m) magnetic induction
ampere-turns / weber A / Wb C2 / (kg m2) Reluctance
henry H V &middot s / A = kg m2 s / (C2) Inductance
henry per metre H / m kg m s / (C2) Permeability
(dimensionless) - - Magnetic susceptibility

Last updated: 02-10-2005 21:17:48
Last updated: 05-03-2005 17:50:55