How Do You Calculate the Laplacian in Polar Coordinates?

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SUMMARY

The discussion focuses on calculating the Laplacian in polar coordinates, specifically in two dimensions. The correct approach involves using the scalar operator $$\nabla ^2=\frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2}$$ and applying the chain rule for partial derivatives. The user emphasizes the importance of correctly handling unit vectors and their derivatives, noting that the process can be tedious but is essential for understanding the concept. The mention of "purple brackets" indicates a common point of confusion that needs clarification.

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davon806
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Homework Statement


I am trying to calculate the laplacian in polar coordinates but I failed.Please see the attached

Homework Equations




The Attempt at a Solution


My solution to this was uploaded in the attached.I was wondering what's wrong with the purple brackets since they shouldn't exist( If you sum A,B,C and D up). Thanks
 

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I don't think you should be messing with unit vectors and their derivatives. The Laplacian is a scalar operator. It looks like you are doing it in two dimensions. Start with $$\nabla ^2=\frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2}$$
Observe that according to the chain rule
$$\frac{\partial}{\partial x}=\frac{\partial r}{\partial x}\frac{\partial}{\partial r}+\frac{\partial \theta}{\partial x}\frac{\partial}{\partial \theta}$$
and one more time for the second partial derivative. Repeat with y and then add the two results.

This is the "brute force" method and involves tedious algebra. It is the kind of thing that one has to do once in one's life to satisfy oneself that one can do it. :smile:
 
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