|Unit system SI derived unit|
Named after Nikola Tesla
In SI base units: kg⋅s⋅A
|Unit of Magnetic field strength|
The tesla (symbol T) is a unit of measurement of the strength of a magnetic field. It is a derived unit of the International System of Units, the modern form of the metric system.
One tesla is equal to one weber per square metre. The unit was announced during the General Conference on Weights and Measures in 1960 and is named in honour of Nikola Tesla, upon the proposal of the Slovenian electrical engineer France Avčin.
The strongest fields encountered from permanent magnets are from Halbach spheres and can be over 4.5 T. The strongest field trapped in a laboratory superconductor as of June 2014 is 21 T. The record magnetic field has been produced by scientists at the Los Alamos National Laboratory campus of the National High Magnetic Field Laboratory, the world's first 100-tesla non-destructive magnetic field.
A particle, carrying a charge of one coulomb, and passing through a magnetic field of one tesla, at a speed of one metre per second, perpendicular to said field, experiences a force with magnitude one newton, according to the Lorentz force law. As an SI derived unit, the tesla can also be expressed as
(The last equivalent is in SI base units).
Units used:A = ampere C = coulomb kg = kilogram m = metre N = newton s = second H = henry V = volt J = joule Wb = weber
Electric vs. magnetic field
In the production of the Lorentz force, the difference between these fields is that a force from a magnetic field on a charged particle is generally due to the charged particle's movement, while the force imparted by an electric field on a charged particle is not due to the charged particle's movement. This may be appreciated by looking at the units for each. The unit of electric field in the MKS system of units is newtons per coulomb, N/C, while the magnetic field (in teslas) can be written as N/(C·m/s). The dividing factor between the two types of field is metres per second (m/s), which is velocity. This relationship immediately highlights the fact that whether a static electromagnetic field is seen as purely magnetic, or purely electric, or some combination of these, is dependent upon one's reference frame (that is, one's velocity relative to the field).
In ferromagnets, the movement creating the magnetic field is the electron spin (and to a lesser extent electron orbital angular momentum). In a current-carrying wire (electromagnets) the movement is due to electrons moving through the wire (whether the wire is straight or circular).
One tesla is equivalent to:10,000 (or 104) G (gauss), used in the CGS system. Thus, 10 kG = 1 T (tesla), and 1 G = 10−4 T. 1,000,000,000 (or 109) γ (gamma), used in geophysics. Thus, 1 γ = 1 nT (nanotesla). 42.6 MHz of the 1H nucleus frequency, in NMR. Thus, the magnetic field associated with NMR at 1 GHz is 23.5 T.
One tesla is equal to 1 V·s/m2. This can be shown by starting with the speed of light in vacuum, c = (ε0μ0)−1/2, and inserting the SI values and units for c (7008299800000000000♠2.998×108 m/s), the vacuum permittivity ε0 (6988884999999999999♠8.85×10−12 A·s/(V·m)), and the vacuum permeability μ0 (6994125660000000000♠12.566×10−7 T·m/A). Cancellation of numbers and units then produces this relation.
For those concerned with low-frequency electromagnetic radiation in the home, the following conversions are needed most:1000 nT (nanotesla) = 1 µT (microtesla) = 10 mG (milligauss), 1,000,000 µT = 1 T.
For the relation to the units of the magnetising field (ampere per metre or oersted), see the article on permeability.