# Polar coordinates, sign ambiguity

1. Mar 11, 2015

The position of a point in cartesian coordinates is given by:
$$\vec{r} = x \hat{\imath} + y \hat{\jmath}$$
In polar coordinates, it is given by:
$$\vec{r} = r \hat{r}$$
Now, $x = r \cos{θ}$ and $y = r \sin{θ}$ assuming $θ$ is measured counterclockwise from the $x$-axis.
Equating the two expressions we obtain:
$$\hat{r} = \hat{\imath} \cos{θ} + \hat{\jmath} \sin{θ}$$
Let's say $\hat{θ} = A \hat{\imath} + B \hat{\jmath}$
We know that:
$$\hat{r} ⋅ \hat{θ} = A\cos{θ} + B\sin{θ} = 0$$
And
$$A^2 + B^2 = 1$$
Since $A = -B \tan{θ}$:
$$B^2 \tan^2{θ} + B^2 = 1$$
$$B^2 = \cos^2{θ}$$
$$B = \pm \cos{θ}$$
And
$$A = \mp \sin{θ}$$
Therefore:
$$\hat{θ} = \mp \sin{θ} \hat{\imath} \pm \cos{θ} \hat{\jmath}$$
On what basis do we choose the correct direction for the tangential unit vector?

2. Mar 11, 2015

### Staff: Mentor

We choose it so that the cross product is positive.

3. Mar 11, 2015

### vanhees71

To use the arctan formula is not a good idea. You have to adjust the angle by hand, corresponding to the quadrant, where your position vector is located. A more handy formula is
$$\theta=\mathrm{sign}\,y \arccos \left(\frac{x}{\sqrt{x^2+y^2}} \right).$$
Then $\theta \in (-\pi,\pi]$.

4. Mar 11, 2015

That is, $\hat{r} × \hat{θ} = \hat{k}$, right?

5. Mar 11, 2015

Where have I used the arctangent function?

6. Mar 11, 2015

### vanhees71

Ah, I misunderstood the question. The point is of course, that you choose the vectors of a curvilinear coordinate system $(q_1,q_2)$ as tangents to the coordinate lines. In the case that these are orthogonal coordinates, one usually also normalizes them. In your case you have $q_1=r$ and $q_2=\theta$. The relation to Cartesian coordinates is, as you write
$$\vec{r}=r \cos \theta \hat{i} + r \sin \theta \hat{j}.$$
Then the tangent vectors are
$$\vec{T}_r=\partial_r \vec{r}=\cos \theta \hat{i} + \sin \theta \hat{j},$$
$$\vec{T}_{\theta} = \partial_{\theta} \vec{r} = -r \sin \theta \hat{i} + r \cos \theta \hat{j}.$$
Obviously $\vec{T}_r \cdot \vec{T}_{\theta}=0$, i.e., we have orthogonal coordinates. Thus one uses the normalized basis vectors
$$\hat{r}=\vec{T}_r/|\vec{T}_r|=\vec{T}_r=\cos \theta \hat{i} + \sin \theta \hat{j},$$
$$\hat{\theta}=\vec{T}_{\theta}/|\vec{T}_{\theta}|=\vec{T}_{\theta}/r = -\sin \theta \hat{i} + \cos \theta \hat{j}.$$
A cross product doesn't make too much sense in the planar case. Also one chooses always the vectors in direction of the tangent vectors to the coordinate lines.

For an orientation preserving transformation from Cartesian to curvilinear coordinates you have to make the order of the curvilinear coordinates such that the Jacobian of the transformation is positive. In the case of polar coordinates, the order $(r,\theta)$ is right, because
$$\mathrm{det} \frac{\partial(x,y)}{\partial(r,\theta)}=\mathrm{det} \begin{pmatrix} \cos \theta & \sin \theta ,\\ -r \sin \theta & \cos \theta \end{pmatrix}=r>0.$$

7. Mar 11, 2015

### Staff: Mentor

Just draw a diagram and you will see that when θ=0, θ hat = j, and when θ=π/2, θ hat = -i.

Chet