A Fermat's spiral or parabolic spiral is a plane curve named after Pierre de Fermat.Its polar coordinate representation is given by
which describes a parabola with horizontal axis.
Fermat's spiral is similar to the Archimedean spiral. But an Archimedean spiral has always the same distance between neighboring arcs, which is not true for Fermat's spiral.
Like other spirals Fermat's spiral is used for curvature continuous blending of curves.
Fermat's spiral with polar equation
can be described in cartesian coordinates by the parametric representation
From the parametric representation and one gets a representation by an equation:
A complete Fermat's spiral (both branches) is a smooth double point free curve, in contrast with the Archimedian and hyperbolic spiral. It divides the plane (like a line or circle or parabola) into two connected regions. But this division is less obvious than the division by a line or circle or parabola. It is not obvious to which side a chosen point belongs.
From vector calculus in polar coordinates one gets the formula
for the polar slope and its angle between the tangent of a curve and the corresponding polar circle (s. diagram).
For Fermat's spiral one gets
Hence the slope angle is monotonely decreasing.
From the formula
for the curvature of a curve with polar equation and its derivatives and one gets the curvature of a Fermat's spiral:
At the origin the curvature is . Hence the complete curve has
The area of a sector of Fermat's spiral between two points and is
After raising both angles by one gets
Hence the area of the region between two neighboring arcs is
only depends on the difference of the two angles, not on the angles themselves.
For the example shown in the diagram, all neighboring stripes have the same area: .
This property is used in electrical engineering for the construction of variable capacitors.
In 1636, Fermat wrote a letterto Marin Mersenne which contains the following special case:
Let , then the area of the black region (see diagram) is half of the area of the circle with radius . The regions between neighboring curves (white, blue, yellow) have the same area Hence:
The length of the arc of Fermat's spiral between two points can be calculated by the integral:
This integral leads to an elliptical integral, which can be solved numerically.
The inversion at the unit circle has in polar coordinates the simple description: .
For both curves intersect at a fixpoint on the unit circle.
In disc phyllotaxis, as in the sunflower and daisy, the mesh of spirals occurs in Fibonacci numbers because divergence (angle of succession in a single spiral arrangement) approaches the golden ratio. The shape of the spirals depends on the growth of the elements generated sequentially. In mature-disc phyllotaxis, when all the elements are the same size, the shape of the spirals is that of Fermat spirals—ideally. That is because Fermat's spiral traverses equal annuli in equal turns. The full model proposed by H Vogel in 1979 is
where θ is the angle, r is the radius or distance from the center, and n is the index number of the floret and c is a constant scaling factor. The angle 137.508° is the golden angle which is approximated by ratios of Fibonacci numbers.
Fermat's spiral has also been found to be an efficient layout for the mirrors of concentrated solar power plants.
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