Magnetic field density

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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. It is a response of a medium to the presence of a magnetic field. The SI unit of magnetic flux density is the tesla. 1 tesla = 1 weber per square metre.

It can be more easily explained if one works 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

Image:Lhr.gif

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' 20 T.

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 adjusted to incorporate moving single charges, ie protons, electrons, and so on via

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. It can also remembered in the following way. From the thumb to second finger, indicating 'Force', 'B-field', and 'I(Current)' respectively. Therefore it is F-B-I in short.

For professional languages, right hand grip rule is used instead which originated from the definition of cross product in the right hand system of coordinates.

Other units of magnetic flux density are