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^[1] 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.^[2]

Definition

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

$\mathrm{1\, T = 1\,\frac{V\cdot s}{m^2} = 1\,\frac{N}{A\cdot m} = 1\,\frac{Wb}{m^2} = 1\,\frac{kg}{C\cdot s} = 1\,\frac{kg}{A\cdot s^2} = 1\,\frac{N\cdot s}{C\cdot m}}$

(in SI base units).^[3]

Units used:

A = ampere

C = coulomb

kg = kilogram

m = meter

N = newton

s = second

T = tesla

V = volt

Wb = weber

Electric vs Magnetic Field

The difference between magnetic field strength (in tesla) vs electric field strength can be confusing.^{[citation needed]}

The difference is that a force of magnetic field on a charged particle is generally due to the charged particle's movement^[4] 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).^[5][6]

In ferromagnets the movement creating the magnetic field is the electron spin^[7] (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).

History

The tesla was announced in honor of the Serbian inventor and electrical engineer Nikola Tesla, during the Conférence Générale des Poids et Mesures in 1960.

Conversions

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.

For the relation to the units of the magnetizing field (amperes per meter or oersteds) see the article on permeability.

Examples

Main article: Orders of magnitude (magnetic field)

• 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^[8]

References

1. ^ "Details of SI units". sizes.com. 2011-07-01. http://www.sizes.com/units/SI.htm. Retrieved 2011-10-04. 
2. ^ "...our data of 5.16 T dipole magnet...", The Strongest Permanent Dipole Magnet
3. ^ The International System of Units (SI), 8th edition, BIPM, eds. (2006), ISBN 92-822-2213-6, Table 3. Coherent derived units in the SI with special names and symbols
4. ^ Gregory, Frederick (2003). History of Science 1700 to Present. The Teaching Company. 
5. ^ Parker, Eugene (2007). Conversations on electric and magnetic fields in the cosmos. Princeton University press. p. 65. http://books.google.com/books?id=7gJ_i3CTcpQC&pg=PA65&dq=reference+frame+electric+magnetic+field#v=onepage&q=reference%20frame%20electric%20magnetic%20field&f=false. 
6. ^ Kurt, Oughstun (2006). Electromagnetic and optical pulse propogation. Springer. p. 81. http://books.google.com/books?id=behRnNRiueAC&pg=PA81&dq=reference+frame+electric+magnetic+field#v=onepage&q=reference%20frame%20electric%20magnetic%20field&f=false. 
7. ^ Herman, Stephen (2003). Delmar's standard textbook of electricity. Delmar Publishers. p. 97. http://books.google.com/books?id=kddgHk0P3NcC&pg=PA97&dq=magnetism+electron+spin#v=onepage&q=magnetism%20electron%20spin&f=false. 
8. ^ "Ultra-High Field". Bruker BioSpin. http://www.bruker-biospin.com/uhf-mri.html. Retrieved 2011-10-04. 

External links

The Wiktionary definition of tesla

• Gauss<->Tesla Conversion Tool