# Frozen-in integral

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The integral of the induction equation of a magnetic field for the limiting case of an ideally-conducting medium. The physical meaning of the frozen-in integral is that by moving the liquid, the force lines of the magnetic field are moved together with the liquid particles.

For the motion of an ideally-conducting medium the magnetic field strength $\mathbf H$ is described by the equation:

$$\frac{d}{dt } \left ( { \frac{\mathbf H} \rho } \right ) = \ \left ( { \frac{\mathbf H} \rho } , \nabla \right ) \mathbf v ,$$

where $\rho$ is the density and $\mathbf v$ is the rate of motion of the medium. A change in the line element $d \mathbf l$ of a force line of the magnetic field is described by the equation

$$\frac{d}{dt } d \mathbf l = ( d \mathbf l , \nabla ) \mathbf v .$$

The vectors $\mathbf H$ and $d \mathbf l$ are collinear:

$${ \frac{\mathbf H} \rho } = \textrm{ const } \cdot d \mathbf l .$$

The following equation, which goes by the name of frozen-in integral, is valid:

$$\frac{\mathbf H d \mathbf l _ {0} } \rho = \ \frac{\mathbf H _ {0} }{\rho _ {0} } d \mathbf l ,$$

where the index "0" refers to the values of the parameters at the initial moment of time.

It follows from the frozen-in integral that the magnetic flux of a field across any surface, encircled by a contour of liquid particles, is independent of time.

How to Cite This Entry:
Frozen-in integral. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Frozen-in_integral&oldid=46996
This article was adapted from an original article by A.P. Favorskii (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article