- Tesla (unit)
The tesla (symbol T) is the SI derived unit of magnetic field B (which is also known as "magnetic flux density"). One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla. The strongest fields encountered from permanent magnets are from Halbach spheres which can be over 5 T.
This SI unit is named after Nikola Tesla. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (T). When an SI unit is spelled out in English, it should always begin with a lower case letter (tesla), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.
A particle carrying a charge of 1 coulomb and passing through a magnetic field of 1 tesla at a speed of 1 meter per second perpendicular to said field experiences a force of 1 newton, according to the Lorentz force law. As an SI derived unit, the tesla can also be expressed as
Electric vs Magnetic Field
The difference between magnetic field strength (in tesla) vs electric field strength can be confusing.
The difference is that a force of 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 can be seen by looking at the units for each. Electric field is N/C, while magnetic field (in tesla) can be written as N/(C*m/s). The difference between the two is m/s, or velocity. This can further be seen by noting that whether a field is magnetic or electric is dependent on one's relativistic 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 current carrying wire (electromagnets) the movement is due to electrons moving through the wire (whether the wire's straight or circular).
1 tesla is equivalent to:
- 10,000 (or 104) G (gauss), used in the CGS system. Thus, 10 G = 1 mT (millitesla), and 1 G = 10−4 T.
- 1,000,000,000 (or 109) γ (gammas), used in geophysics. Thus, 1 γ = 1 nT (nanotesla)
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
Because the tesla is so large in regards to everyday usage, common engineering practice is to report the strength of magnets in Gauss. Scientists are split on this issue, with some insisting on proper SI units at all times and some allowing for more practical labeling.
- 31 µT (3.1×10−5 T) - strength of Earth's magnetic field at 0° latitude (on the equator)
- 5 mT - the strength of a typical refrigerator magnet
- 1.25 T - magnetic field intensity at the surface of a neodymium magnet
- 1 T to 2.4 T - coil gap of a typical loudspeaker magnet
- 1.5 T to 3 T - strength of medical magnetic resonance imaging systems in practice, experimentally up to 17 T
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SI units Base units Derived units Accepted for use
See alsoBook:International System of Units · Category:SI base units
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