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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
SI units
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