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 per square metre.
It can be more easily explained if one works backwards from the equation: 
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' 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.
Other units of magnetic flux density are
- 1 gauss = 10-4 teslas = 100 microteslas (µT)
- 1 gamma = 10-9 teslas = 1 nanotesla (nT)
SI units
