Problem 1B-4*: Transformation from two-dimensional rectangular coordinate representation to polar coordinate representation and the transformation Jacobian

A two dimensional position vector can be expressed in the rectangular coordinate representation as (x, y) or in the polar coordinate representation as (r, θ).

(a) Derive the relations between the (x, y) pair and the (r, θ) pair.

(b) An integral expressed in the rectangular coordinate representation can be transformed into the polar coordinate representation as

       
Find the Jacobian, J, of this transformation.






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Solution:

(a) The relation between the rectangular representation of a position vector (x, y) and the polar coordinate representation (r, θ) is shown in the right picture. From the picture we see that


(b) From Eq. (1) we have

        
We know from calculus that the Jacobian, J, for the transformation from (x, y) to (r, θ) representation is the determinant
        
Thus
        

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