Oersted

For other uses, see Oersted (disambiguation).

Oersted (abbreviated as Oe) is the unit of magnetizing field (also known as H-field, magnetic field strength or intensity) in the CGS system of units.

Difference between cgs and SI systems

In cgs the unit of H-field is the oersted and the unit of B-field is the gauss. In the SI system, Ampere per meter (A/m) is used for H-field and Tesla is used for B-field.^[1]

History

The unit was established by the IEC in 1930 ^[2] in honour of the Danish physicist Hans Christian Ørsted.

Definition

The oersted is defined as an dyne per unit pole.^{[citation needed]} The Oersted is 1000/4π (≈79.5774715) ampere per meter, in terms of SI units.^[3][4][5][6]

So, the H-field strength inside a long solenoid wound with 79.58 turns per meter of a wire carrying 1 A will be approximately 1 oersted. The preceding statement is exactly correct if the solenoid considered is infinite in length and the current evenly distributed over its surface.

The oersted is closely related to the gauss, the CGS unit of magnetic flux density. In a vacuum, if the magnetizing field strength is 1 Oe, then the magnetic field density is 1 Gs, whereas in a medium having permeability μ_r (relative to permeability of vacuum), their relation is:

B(G) = μ_r * H(Oe)

Because oersteds are used to measure magnetizing field strength, they are also related to magnetomotive force (mmf) of current in a single-winding wire-loop:

$H(\mbox{Oe})= \frac{1000}{4 \pi} \frac{I(\mbox{A})}{l(\mbox{m})}$

The stored energy in a magnet, called magnet performance or maximum energy product (often abbreviated BH_max), is typically measured in units of megagauss-oersteds (MGOe). One MGOe is approximately equal to 7957.74715 J/m³.^[7]

See also

• Centimetre gram second system of units
• Ampere's model of magnetization

References

1. ^ Kaye, G.W.C & Laby, T.H:Table of Physical and Chemical Constants, page 14. Longman, 1973.
2. ^ IEC history
3. ^ Magnetic Conversion Factors
4. ^ EMF Fundamentals
5. ^ [e-mail]
6. ^ Derived CGS Units with Special Names
7. ^ eFunda: Glossary: Units: Energy Density Units: Megagauss-Oersted (MGOe)