Difference between revisions of "Oersted (unit)"

From Citizendium
Jump to: navigation, search
(Conversion to SI units)
 
(13 intermediate revisions by the same user not shown)
Line 1: Line 1:
 
{{subpages}}
 
{{subpages}}
The '''oersted''' (abbreviated as Oe) is  the unit of [[magnetic field]] strength |'''H'''| in the
+
In [[physics]], the '''oersted''' (symbol '''Oe''') is  the unit of [[magnetic field]] strength |'''H'''| in the emu ( electromagnetic unit) and [[Gaussian units|Gaussian system]]s of units. Both systems are cgs (centimeter-gram-second) systems.
cgs-emu (centimeter-gram-second electromagnetic unit) and Gaussian systems of units. The field strength in a point in vacuum is 1 Oe if a unit magnetic pole in the point experiences a force of 1 [[dyne]] ( = 1&sdot;10<sup>&minus;5</sup> newton).  
+
  
The  [[Biot-Savart law#Magnetic field on axis of circular current|Biot-Savart law]]
+
The relation to the corresponding [[SI]] unit (ampere times turn per meter) is
in Gaussian units,  for which in vacuum '''B''' = '''H''',  states that the field in the center of a conducting loop of radius ''r'' is,  
+
:1 Oe = 1000/4π&nbsp;A&sdot;turn/m.
 +
See [[solenoid]] for the explanation of the SI unit, which is one of the few electromagnetic units that carries no name. 
 +
 
 +
The oersted is named for the Danish scientist [[Hans Christian Oersted]].
 +
 
 +
==Definition==
 +
The definition of Oe follows from the [[Biot-Savart law#Infinite straight conductor|Biot-Savart law]] giving the field |'''H'''| due to an electric current ''I'' in an infinitely long straight conductor,
 +
:<math>
 +
|\mathbf{H}| = \begin{cases}
 +
{\displaystyle \frac{2I}{4\pi\, r}}& \quad \hbox{SI units} \\ \\
 +
{\displaystyle \frac{2I}{c r}}& \quad \hbox{Gaussian units}\\
 +
\end{cases}
 +
</math>
 +
where ''r'' is the distance of the field point to the conductor and ''c'' is the [[speed of light]] (&asymp; 3&sdot;10<sup>10</sup> cm/s).
 +
In the SI definition ''r'' is in meters, whereas in Gaussian units ''r'' is in centimeters.
 +
 +
For Gaussian units, the oersted is defined as ''the strength of the magnetic field at a distance of 1 centimeter from a straight conductor of infinite length and negligible circular cross section that carries a current  of &frac12;&sdot;''c''&nbsp;statA ([[statampere]])''.
 +
 
 +
==Conversion to SI units==
 +
Because  the speed of light ''c'' in two different units is needed, we write ''c'' numerically, but in the approximate form 3·10<sup>8</sup> m/s or 3·10<sup>10</sup> cm/s. Since the approximate forms cancel, the end result is exact. In order to explain the factor 1000/4π, relating Gaussian and SI units,  consider an infinite wire carrying a current of &frac12;&sdot;3&sdot;10<sup>10</sup>&nbsp; statA and measure |'''H'''| at one cm distance from the wire. By definition the field has the strength  1 Oe.  Note that the current  is equal to
 +
: ''I'' = &frac12;&sdot;3&sdot;10<sup>10</sup> statA = &frac12;&sdot;3&sdot;10<sup>10</sup> A/(3&sdot;10<sup>9</sup>) = &frac12;&sdot;10 A,
 +
 
 +
Applying the Biot-Savart equation valid in SI units (current ''I'' in A, distance ''r'' in m), we find that the field in the same physical setup, but expressed in A&sdot;turn/m is
 
:<math>
 
:<math>
|\mathbf{H}| = \frac{2\pi i}{c r},
+
|\mathbf{H}| = \frac{2 \,(\tfrac{1}{2}\cdot 10)}{4\pi \, 10^{-2}} = \frac{1000}{4\pi},
 
</math>
 
</math>
where ''c'' is speed of light. Hence, one may alternatively define the Gaussian unit ''oersted'' as the magnetic field strength |'''H'''| in the center of a conducting loop with radius of 1 cm, carrying an electric current ''i'' of 1/(2&pi;) ampere. Note here that  ampere/''c'' (the ''statampere'') is the unit of current and the centimeter is the unit of length  in the  Gaussian system of units.
+
from which follows that a field of strength 1 Oe has strength <math>\scriptstyle \frac{1000}{4\pi}</math>&nbsp; A·turn/m exactly. Since "turn" (number of turns) is a dimensionless number, it is often omitted in the SI unit, which then becomes simply A/m.
  
One oersted equals 1000/4π A/m &nbsp; (ampere per meter, which is  the [[SI]] unit for |'''H'''|).  
+
==Notes==
 +
1. Before 1930 there was much confusion about the difference between the [[gauss (unit)|gauss]] (the Gaussian unit of [[magnetic flux density]] '''B''') and the oersted.  At its meeting in Stockholm in 1930 the Advisory Committee on Nomenclature of the International Electrotechnical Commission eliminated all ambiguity by adopting the gauss for the unit of [[magnetic flux density]] and the oersted for the unit of magnetic field strength.  The gauss is defined through a time-dependent change in magnetic flux density and the oersted is defined through the field created by an electric current.
  
The oersted is named after the Danish physicist [[Hans Christian Oersted]]. Before 1932 the oersted was known as gauss, a name sometimes still applied, though now more properly used for the cgs-emu and Gaussian unit of  strength of [[magnetic induction]] |'''B'''|.
+
2. The historically oldest definition of the oersted is: The magnetic field strength |'''H'''| in a point in vacuum is 1 Oe, if a unit magnetic pole in the point experiences a force of 1 [[dyne]] ( = 1&sdot;10<sup>&minus;5</sup> newton). Because a magnetic pole does not exist in nature and must be realized by a long bar magnet, this definition was not practicable and is now obsolete.

Latest revision as of 16:53, 21 September 2009

This article is developing and not approved.
Main Article
Discussion
Related Articles  [?]
Bibliography  [?]
External Links  [?]
Citable Version  [?]
 
This editable Main Article is under development and subject to a disclaimer.

In physics, the oersted (symbol Oe) is the unit of magnetic field strength |H| in the emu ( electromagnetic unit) and Gaussian systems of units. Both systems are cgs (centimeter-gram-second) systems.

The relation to the corresponding SI unit (ampere times turn per meter) is

1 Oe = 1000/4π A⋅turn/m.

See solenoid for the explanation of the SI unit, which is one of the few electromagnetic units that carries no name.

The oersted is named for the Danish scientist Hans Christian Oersted.

Definition

The definition of Oe follows from the Biot-Savart law giving the field |H| due to an electric current I in an infinitely long straight conductor,

where r is the distance of the field point to the conductor and c is the speed of light (≈ 3⋅1010 cm/s). In the SI definition r is in meters, whereas in Gaussian units r is in centimeters.

For Gaussian units, the oersted is defined as the strength of the magnetic field at a distance of 1 centimeter from a straight conductor of infinite length and negligible circular cross section that carries a current of ½⋅c statA (statampere).

Conversion to SI units

Because the speed of light c in two different units is needed, we write c numerically, but in the approximate form 3·108 m/s or 3·1010 cm/s. Since the approximate forms cancel, the end result is exact. In order to explain the factor 1000/4π, relating Gaussian and SI units, consider an infinite wire carrying a current of ½⋅3⋅1010  statA and measure |H| at one cm distance from the wire. By definition the field has the strength 1 Oe. Note that the current is equal to

I = ½⋅3⋅1010 statA = ½⋅3⋅1010 A/(3⋅109) = ½⋅10 A,

Applying the Biot-Savart equation valid in SI units (current I in A, distance r in m), we find that the field in the same physical setup, but expressed in A⋅turn/m is

from which follows that a field of strength 1 Oe has strength   A·turn/m exactly. Since "turn" (number of turns) is a dimensionless number, it is often omitted in the SI unit, which then becomes simply A/m.

Notes

1. Before 1930 there was much confusion about the difference between the gauss (the Gaussian unit of magnetic flux density B) and the oersted. At its meeting in Stockholm in 1930 the Advisory Committee on Nomenclature of the International Electrotechnical Commission eliminated all ambiguity by adopting the gauss for the unit of magnetic flux density and the oersted for the unit of magnetic field strength. The gauss is defined through a time-dependent change in magnetic flux density and the oersted is defined through the field created by an electric current.

2. The historically oldest definition of the oersted is: The magnetic field strength |H| in a point in vacuum is 1 Oe, if a unit magnetic pole in the point experiences a force of 1 dyne ( = 1⋅10−5 newton). Because a magnetic pole does not exist in nature and must be realized by a long bar magnet, this definition was not practicable and is now obsolete.