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单词 PolarCoordinates
释义

polar coordinates


Let x,y be Cartesian coordinatesMathworldPlanetmath for 2.

Then r0, θ[0,2π) related to (x,y) by

x(r,θ)=rcosθ,
y(r,θ)=rsinθ,

are the polar coordinates for (x,y). It is simply written (r,θ).

The polar coordinates of Cartesian coordinates(x,y)2{0} are

r(x,y)=x2+y2,
θ(x,y)=arctan(x,y),

where arctan is defined here (http://planetmath.org/OperatornamearcTanWithTwoArguments).

Polar basis.Polar coordinates are equipped with an orthonormal base {𝐞𝐫,𝐞θ}, which can be defined in terms of the standard cartesian base {𝐢,𝐣} in 2 as follows.

[𝐞𝐫𝐞θ]=[cosθ𝐢+sinθ𝐣-sinθ𝐢+cosθ𝐣],

where 𝐞𝐫,𝐞θ are so-called radial and traverse or angular vector, respectively. Since these vectors are variable in direction, they are differentiableMathworldPlanetmathPlanetmath. In fact,

[d𝐞𝐫dθd𝐞θdθ]=[𝐞θ-𝐞𝐫].

The geometrical action of the derivativePlanetmathPlanetmath operator d/dθ is like a rotation operator that rotates each base vector a counter-clockwise angle equals to π/2.

Position vector. For an arbitrary point of polar coordinates (r,θ), its position vector comes given by the single equation

𝐫=r𝐞𝐫.

RelationsMathworldPlanetmathPlanetmath with complex numbers.When the Euclidean planeMathworldPlanetmath 2 is identified with by

(x,y)x+yi,

it is possible to define multiplications on 2.  Via polar coordinates, the formulaMathworldPlanetmathPlanetmath for this multiplication becomes very simple, thanks to Euler’s formula (http://planetmath.org/EulerRelation)

cosθ+isinθ=eiθ.

Thus, we have the following identification:

(r,θ)(x,y)x+yi=rcosθ+(rsinθ)i=reiθ.

If P=(r1,θ1)and Q=(r2,θ2), the productPlanetmathPlanetmath of P and Q works out to be(r1r2,θ1+θ2).(Even if one is not familiar with the complex exponentialPlanetmathPlanetmath, this assertion may be checkeddirectly using the angle sum identities for cos and sin.)

Multiplications of polar coordinates have some simple geometricinterpretationsMathworldPlanetmathPlanetmath. For example, if R=(1,α) and Q=(r,β),then QRQ given by(1,α)(r,β)=(r,α+β) is the rotationMathworldPlanetmath of Q byangle α. If S=(t,0), then (t,0)(r,β)=(tr,β) canbe viewed as the scalingMathworldPlanetmath of Q along the ray OQby t. Note also that multiplication by (t,0) has the same effectas multiplication by the scalar t.

For more on polar coordinates, including their construction and extensionsPlanetmathPlanetmath on domain of polar coordinates r and θ, see here (http://planetmath.org/ConstructionOfPolarCoordinates).

Calculus in polar coordiantes.For reference, here are some formulae for computing integrals and derivativesin polar coordinates. The JacobianMathworldPlanetmathPlanetmath for transforming from rectangular topolar cordinates is

(x,y)(r,θ)=r

so we may compute the integral of a scalar field f as

f(r,θ)r𝑑r𝑑θ.

Partial derivativeMathworldPlanetmath operators transform as follows:

x=cosθr-1rsinθθ
y=sinθr+1rcosθθ
r=cosθx+sinθy
θ=-rsinθx+rcosθy
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更新时间:2025/5/4 10:22:10