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:

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

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

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

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