Biot-Savart law: Difference between revisions

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In [[physics]], more particularly in [[electrodynamics]], the law first formulated by [[Jean-Baptiste Biot]] and [[Felix Savart|Félix Savart]] <ref>J.-B. Biot and F. Savart, ''Note sur le Magnétisme de la pile de Volta,'' Annales Chim. Phys. vol. '''15''', pp. 222-223 (1820)</ref> describes the [[magnetic induction]] '''B''' (proportional to the magnetic field '''H''') caused by a direct electric current in a wire. Biot and Savart interpreted their measurements by an integral relation. [[Laplace]] gave a differential form of their result, which now often is also referred to  as the Biot-Savart law, or sometimes as the Biot-Savart-Laplace law. By integrating Laplace's equation over an infinitely long wire, the original integral form of Biot and Savart is obtained.
In [[physics]], more particularly in [[electrodynamics]], the law first formulated by [[Jean-Baptiste Biot]] and [[Felix Savart|Félix Savart]] <ref>J.-B. Biot and F. Savart, ''Note sur le Magnétisme de la pile de Volta,'' Annales Chim. Phys. vol. '''15''', pp. 222-223 (1820)</ref> describes the [[magnetic induction]] '''B''' (proportional to the magnetic field '''H''') caused by a direct electric current in a wire. Biot and Savart interpreted their measurements by an integral relation. [[Laplace]] gave a differential form of their result, which now often is also referred to  as the Biot-Savart law, or sometimes as the Biot-Savart-Laplace law. By integrating Laplace's equation over an infinitely long wire, the original integral form of Biot and Savart is obtained.


==A little history==
==Ørsted's discovery==
[[Image:Oersted.png|left|thumb|350px|Ørsted's experiment: A bar magnet is hung above a wire carrying the direct current ''i''.  The magnetic field  '''B''' is in the direction of the turn of a screw driven into the direction of ''i''. The magnet experiences a torque, the south pole moves toward the reader, the north pole away from the reader.  ]]
[[Image:Oersted.png|left|thumb|350px|Ørsted's experiment: A bar magnet is hung above a wire carrying the direct current ''i''.  The magnetic field  '''B''' is in the direction of the turn of a screw driven into the direction of ''i''. The magnet experiences a torque, the south pole moves toward the reader, the north pole away from the reader.  ]]
Electricity and magnetism have been known since Antiquity. Rubbing amber was known to give sparks, and lodestone needles have been used as magnetic compasses since the Middle Ages. However, the two phenomena, electricity and magnetism, were seen as unrelated. It took [[Alessandro Volta]]'s discovery of the battery ("Voltaic pile") before electric currents of  non-negligible strength could be generated. Soon after Volta's discovery (around 1800), the Danish physicist [[Hans Christian Oersted|Ørsted]] noticed in April 1820, while experimenting with the Voltaic pile, an effect of an electric  current on a permanent magnet. He wrote a (Latin) publication which he sent round Europe.
Electricity and magnetism have been known since Antiquity. Rubbing amber was known to give sparks, and lodestone needles have been used as magnetic compasses since the Middle Ages. However, the two phenomena, electricity and magnetism, were seen as unrelated. It required [[Alessandro Volta]]'s invention of the battery ("Voltaic pile") before electric currents of  non-negligible strength could be generated. Soon after Volta's invention (around 1800), the Danish physicist [[Hans Christian Oersted|Ørsted]] noticed in April 1820, while experimenting with the Voltaic pile, an effect of an electric  current on a permanent magnet. He wrote a (Latin) publication which he sent round Europe.
[[François Arago]] talked about the discovery at the French Académie des Sciences, where it was picked up by [[Jean-Baptiste Biot]] and [[André-Marie Ampère]]. Biot and his coworker Savart  
[[François Arago]] talked about the discovery at the French Académie des Sciences, where it was picked up by [[Jean-Baptiste Biot]] and [[André-Marie Ampère]]. Biot and his coworker Savart  
made quantitative measurements and published, still in 1820, a short note. Laplace generalized their result.
made quantitative measurements and published, still in 1820, a short note. Laplace generalized their result mathematically.


Although Ørsted, other contemporary physicists, the general public, and subsidizing politicians had no way of foreseeing this, Ørsted's very fundamental discovery led to the invention of the [[dynamo]] and the [[electric motor]]. The importance of these discoveries does not need to be emphasized. For instance, today every western household has a couple of dozen electromotors in and around the house (fans, vacuum cleaners, electric mower, CD player, electric can opener, almost all utensils have an electromotor).
Although Ørsted himself could not have foreseen this, his very fundamental discovery led soon to the invention of the [[dynamo]] and the [[electric motor]]. The importance of these inventions cannot be exaggerated. To mention just one aspect: every western household today has a couple of dozen electromotors in and around the house.


==Laplace's formula==
==Laplace's formula==

Revision as of 03:45, 10 February 2008

In physics, more particularly in electrodynamics, the law first formulated by Jean-Baptiste Biot and Félix Savart [1] describes the magnetic induction B (proportional to the magnetic field H) caused by a direct electric current in a wire. Biot and Savart interpreted their measurements by an integral relation. Laplace gave a differential form of their result, which now often is also referred to as the Biot-Savart law, or sometimes as the Biot-Savart-Laplace law. By integrating Laplace's equation over an infinitely long wire, the original integral form of Biot and Savart is obtained.

Ørsted's discovery

Ørsted's experiment: A bar magnet is hung above a wire carrying the direct current i. The magnetic field B is in the direction of the turn of a screw driven into the direction of i. The magnet experiences a torque, the south pole moves toward the reader, the north pole away from the reader.

Electricity and magnetism have been known since Antiquity. Rubbing amber was known to give sparks, and lodestone needles have been used as magnetic compasses since the Middle Ages. However, the two phenomena, electricity and magnetism, were seen as unrelated. It required Alessandro Volta's invention of the battery ("Voltaic pile") before electric currents of non-negligible strength could be generated. Soon after Volta's invention (around 1800), the Danish physicist Ørsted noticed in April 1820, while experimenting with the Voltaic pile, an effect of an electric current on a permanent magnet. He wrote a (Latin) publication which he sent round Europe. François Arago talked about the discovery at the French Académie des Sciences, where it was picked up by Jean-Baptiste Biot and André-Marie Ampère. Biot and his coworker Savart made quantitative measurements and published, still in 1820, a short note. Laplace generalized their result mathematically.

Although Ørsted himself could not have foreseen this, his very fundamental discovery led soon to the invention of the dynamo and the electric motor. The importance of these inventions cannot be exaggerated. To mention just one aspect: every western household today has a couple of dozen electromotors in and around the house.

Laplace's formula

Magnetic induction dB at point r due to infinitesimal piece ds (red) of wire (blue) transporting electric current i.

The infinitesimal magnetic induction at point (see figure on the right) is given by the following formula due to Laplace,

where the magnetic induction is given as a vector product, i.e., is perpendicular to the plane spanned by and . The electric current i is constant in time. The piece of the wire contributing to the magnetic induction can be seen as a vector of infinitesimal length ds and with direction tangent to the wire. The constant k depends on the units chosen. In rationalized SI units k is the magnetic constant (vacuum permeability) divided by 4π. The magnetic constant μ0 = 4π ×10−7 N/A2 (newton divided by ampere squared). In Gaussian units k = 1 / c (one over the velocity of light).

If we remember the fact that the vector r has dimension length, we see that this equation is an inverse distance squared law.

Formula of Biot and Savart

Field B due to current i in infinitely long straight wire.

Take a straight infinitely long wire transporting the current i. Write, using R = rsinα (see the figure),

where is a unit vector perpendicular to the plane spanned by the wire and the vector perpendicular to the wire. Note that if moves along the wire all contributions from the segments to the magnetic induction are along this unit vector. Hence, if we integrate over the wire we add up all these contributions, so that

where, by the Pythagorean theorem,

Substition of y = s / R and y = cotφ = cosφ / sinφ, successively, gives

where i is the current and R the distance of the point of observation of the magnetic induction to the wire. The constant k depends on the choice of electromagnetic units and is 10−7 henry/m [= Vs/A = N/A2] in rationalized SI units. This equation gives the original formulation of Biot and Savart. The SI dimension of B is T [tesla: 1 T = 1 N/(Am), newton divided by ampere meter]. A field of 1 T (SI) corresponds to 10000 gauss (cgs units).

Generalized Biot-Savart-Laplace law

From hereon vectors are indicated by bold letters, arrows on top are omitted.

In the above we wrote i for the current, which is equal to the current density J times the cross section A of the wire. If the current density J is not constant over the cross section, i.e., J = J(r' ), we must use an integral over the cross section A. Rather than introducing a surface element we multiply immediately by ds and obtain an infinitesimal volume element ,

where we assumed that the current density is a vector parallel to the segment ds and where the volume element has height ds and base an infinitesimal element of A. The B-field at point r, due to a volume V = As of the current becomes,

Note that:

where we choose the nabla operator to act on the unprimed coordinates and hence it may be moved outside the integral, giving the following generalized form of the Biot-Savart law for the magnetic induction at point r:

where V is a finite volume segment of the current generating the B-field. The total B-field is obtained by having V encompass all current.

Maxwell equations

We will show that the expression for B given in Eq. (1) satisfies the Maxwell equations. This is of interest, since the Maxwell equations can be seen as a set of postulates for classical electrodynamics. We will show that the Biot-Savart law is a consequence of the postulates (although Biot and Savart made their discovery some forty-five years before Maxwell formulated his equations).

Since it is known from vector analysis that

for any vector field V(r), it is immediately clear that B satisfies the following Maxwell equation:

The one other Maxwell equation of interest is

where we assumed that there are no time-dependent electric fields present. In SI units 4πk is equal to the vacuum permeability μ0. In the proof we will need the following results

The first equation is well-known in vector analysis, the second follows by differentiating to the components of r and r' and equating. The third equation has a Dirac delta function on the right-hand side and follows from distribution theory. The last equation follows from charge density (ρ) conservation in conjunction with the continuity equation,

Now

In the first term of the last step we replaced the unprimed nabla by a primed one (times minus) and then applied a turn-over rule with the primed nabla. This turn-over rule can be justified by partial integration and gives a minus sign, or, in other words, nabla is an anti-hermitian operator. Using that the divergence of the current density vanishes and the defining property of the delta function, we get

This shows that B(r) of Eq. (1) (the generalized Biot-Savart law) satisfies this Maxwell equation.

References

  1. J.-B. Biot and F. Savart, Note sur le Magnétisme de la pile de Volta, Annales Chim. Phys. vol. 15, pp. 222-223 (1820)

Further reading:

  • J. D. Jackson, Classical Electrodynamics, 2nd edition, John Wiley, New York (1975).