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: <math>B=\frac {F} {I L} \,<math>

where

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

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 <math>F=BQv<math>

where

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

• 1 Gauss = 10^-4 Teslas
• 1 gamma = 10^-9 Teslas

SI units

de:Magnetische Flussdichte ja:磁束密度 nl:Magnetische fluxdichtheid sl:gostota magnetnega polja pt:Densidade do fluxo magn�tico