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A '''magnetic field''' is a [[vector]] field responsible for generating forces on electrically charged objects and magnets. Magnetic fields are generated by magnetic [[dipole]]s, moving [[electric charge]]s, or changing [[electric field]]s.
A '''magnetic field''' is a [[vector field]] responsible for generating [[force]]s on electrically charged objects and magnets. Magnetic fields are generated by magnetic [[dipole]]s, moving [[electric charge]]s, or changing [[electric field]]s.
Magnetic fields are linked to electric fields; [[light]], for instance, is a propagating electric and magnetic [[wave]]. Relativistically, a magnetic force in one [[inertial frame]] corresponds to an electric force in another.
Magnetic fields are related to electric fields; together they are part of [[electromagnetism]]. For instance, [[light]] is a propagating electric and magnetic [[wave]]. Relativistically, a magnetic force in one [[inertial frame of reference]] corresponds to an electric force in another.
==Mathematics==
Magnetic fields are often denoted using the letter B. The magnetic force acting on a particle travelling at a [[velocity]] v can be written as:
<math>
\vec{F} = q \left( \vec{v} \times \vec{B} \right)
</math>
where q is the electrical charge on the particle.<ref>[http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/funfor.html#c3 Magnetic force on hyperphysics]</ref> The [[cross product]] means that the force will always act at a right angle to both the field and the velocity and if the velocity is [[parallel]] to the field, no force will act. As the force always acts perpendicular to the velocity, a charged particle in a uniform (constant) magnetic field will undergo [[circular motion]]. The [[magnitude]] of the force can also be written as:
<math>
F = qvB \sin{\theta}
</math>
where θ is the angle between the velocity and magnetic field. The direction can then be found using the [[right hand rule]].
===Force on a Current Carrying Wire===
An electrical [[current]] is rate of flow of charge past a point. By considering a wire with length L, the equation IL=nqv can be derived, where I is the current flowing along that wire and n is the "number density" (the number of charge carriers per unit length). This means that the force on a straight current carrying wire in a '''uniform''' magnetic field can be found as:
<math>
F = BIL \sin{\theta}
</math>
with θ now being the angle between the wire and the magnetic field.
===Magnetic Field Produced by a Current Carrying Wire===
Any electrical current creates a magnetic field. The magnetic field around a straight wire circulates around the wire. Its strength at a point depends only on the perpendicular distance to the wire and the size of the current:
<math>
B = \frac{\mu_0 I}{2 \pi r}
</math>
where r is the distance to the wire and μ<sub>0</sub> is the [[constants|permeability of free space]].
==Earth's Magnetic Field==
==Earth's Magnetic Field==
[[Image:magnetic_field.gif|right|thumb|The View of "Answers in Genesis"]]
[[Image:magnetic_field.gif|right|thumb|Change in the Earth's magnetic field according to Humphreys' model ]]
The magnetic field of [[Earth]] is directional nearly north-to-south, although slightly askance, meaning that "magnetic north" is not the same as "true north," and a person who is orienteering must take into account this change of declination, although it is truly only marginally relevant, unless you are close to either pole.<ref>Boy Scout Handbook.</ref> This field has been decaying at a rapid rate of about about 5% per century, which casts doubt on the theory that the [[Earth]] is billions of years old.<ref>K.L. McDonald and R.H. Gunst, 'An analysis of the earth’s magnetic field from 1835 to 1965,’ ESSA Technical Report, IER 46-IES 1, U.S. Govt. Printing Office, Washington, 1967.</ref> This decay suggests that, at some point, the poles will invert.
The magnetic field of [[Earth]] is directional nearly north-to-south, although slightly askance, meaning that "magnetic north" is not the same as "true north," and a person who is orienteering must take into account this change of declination, although it is truly only marginally relevant, unless you are close to either pole.<ref>Boy Scout Handbook.</ref> This field has been decaying at a rapid rate of about about 5% per century, which casts doubt on the theory that the [[Earth]] is billions of years old.<ref>K.L. McDonald and R.H. Gunst, 'An analysis of the earth’s magnetic field from 1835 to 1965,’ ESSA Technical Report, IER 46-IES 1, U.S. Govt. Printing Office, Washington, 1967.</ref> This decay suggests that, at some point, the poles will invert.
Scientists have speculated about the history of Earth's magnetic field. One group that makes use of the [[Bible]] as a resource for science suggests that the history of the Earth's magnetic field is as depicted to the right.<ref>http://www.answersingenesis.org/creation/v20/i2/magnetic.asp</ref>
Scientists have speculated about the history of Earth's magnetic field. One group that makes use of the [[Bible]] as a resource for science suggests that the history of the Earth's magnetic field is as depicted to the right.<ref name=answersingenesis>[https://answersingenesis.org/astronomy/earth/the-earths-magnetic-field-and-the-age-of-the-earth/ answersingenesis.org]</ref> The model, created by [[Russell Humphreys]], can be summarised as:
* Creation of the Earth's magnetic field when God created the Earth
* A decay in its strength until the time of the [[Great Flood]]
* A series of very rapid reversals during the Flood, perhaps once every one or two weeks<ref name=answersingenesis/>
* Fluctuations continue for around two thousand years after the Flood
* The steady decay of the field resumes
This model suggests the magnetic field of the [[Earth]] must be less than 10,000 years old
== References ==
== References ==
{{reflist}}
==See also==
*[[Electric field]]
*[[Electromagnetism]]
*[[Maxwell's equations]]
[[Category:Physics]]
[[Category:Physics]]
[[Category:Electromagnetism]]