# How do I express an equation in Polar coordinates as a Cartesian one.

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## Summary:

I need to convert an equation that is on polar coordinates into a cartesian but as soon I start doing that I got confused and I'm not really shure about what to do.
I got a polar function.

$$\psi = P(\theta )R(r)$$

When I calculate the Laplacian:

$$\ \vec \nabla^2 \psi = P(\theta)R^{\prime\prime}(r) + \frac{P(\theta)R^{\prime}(r)}{r} + \frac{R(r)P^{\prime\prime}(\theta)}{r^{2}}$$

Now I need to convert this one into cartesian coordinates and then it results very difficult to me because I know how to convert simple equations using the simple relations:

$$x = r Cos(\theta )$$

$$y = r Sin(\theta )$$

$$r= \sqrt{x^{2}+y^{2}}$$

$$\theta= arcTan( \frac{y}{x})$$

I can't figure out how to use this relations in order to replace them in my functions R and P since there are first and second derivatives of a functions dependent on x and y, so I can not just replace the relations (At least not directly).

What I need to do is to express the Laplacian of psi in cartesian coordinates.

Is there a way just to replace in them as :

$$P(\theta) = F(x,y)$$
$$P^{\prime}(\theta) = G(x,y)$$

$$P^{\prime\prime}(\theta) = H(x,y)$$

Or how should I try?

Thanks A lot for your help

## Answers and Replies

mfb
Mentor
If you don't know P and R, the best you can do is setting up something like ##\displaystyle \frac{\partial P(\theta(x,y))}{\partial \theta}## and then applying the chain rule. It will still look quite awkward.