Magnetic field density

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

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

SI electromagnetism units edit
Name	Symbol	Dimensions	Quantity
ampere (SI base unit)	A	A	Current
coulomb	C	A·s	Electric charge, Quantity of electricity
volt	V	J/C = kg·m2·s−3·A−1	Potential difference
ohm	Ω	V/A = kg·m2·s−3·A−2	Resistance, Impedance, Reactance
ohm metre	Ω·m	kg·m3·s−3·A−2	Resistivity
watt	W	V·A = kg·m2·s−3	Electrical power
farad	F	C/V = kg−1·m−2·A2·s4	Capacitance
farad per metre	F/m	kg−1·m−3·A2·s4	Permittivity
reciprocal farad	F−1	kg1·m2·A−2·s−4	Elastance
siemens	S	Ω−1 = kg−1·m−2·s3·A2	Conductance, Admittance, Susceptance
siemens per metre	S/m	kg−1·m−3·s3·A2	Conductivity
weber	Wb	V·s = kg·m2·s−2·A−1	Magnetic flux
tesla	T	Wb/m2 = kg·s−2·A−1	Magnetic flux density
ampere per metre	A/m	m−1·A	magnetic induction
ampere-turns per weber	A/Wb	kg−1·m−2·s2·A2	Reluctance
henry	H	Wb/A = V·s/A = kg·m2·s−2·A−2	Inductance
henry per metre	H/m	kg·m·s−2·A−2	Permeability
(dimensionless)	χ	-	Magnetic susceptibility

See also: Magnetic field density, Admittance, Ampere, Capacitance, Coulomb, Current (electricity), Electric charge, Electric conductance