diff --git a/src/2_coordinates.md b/src/2_coordinates.md index 15cd685cb0cd80e5153b611d251f15e14c81da74..4d946fa88550e9bbe169bdc6cc45daff9373bbac 100644 --- a/src/2_coordinates.md +++ b/src/2_coordinates.md @@ -359,6 +359,7 @@ We have discussed four different coordinate systems: $${\bf r} = (r, \phi).$$ This system can be used in two dimensions. It is particularly suitable for systems with circular symmetry or functions given in terms of these coordinates. + Infinitesimal distance: $$ds^2 = dr^2 + r^2 d\phi^2.$$ Infinitesimal area: $$dA = r dr d\varphi.$$ @@ -366,6 +367,7 @@ We have discussed four different coordinate systems: $${\bf r} = (r, \phi, z).$$ This system can be used in three dimensions. It is particularly suitable for systems with axial symmetry or functions given in terms of these coordinates. + Infinitesimal distance: $$ds^2 = dr^2 + r^2 d\phi^2 + dz^2.$$ Infinitesimal volume:: $$dV = r dr d\varphi dz.$$ @@ -373,6 +375,7 @@ We have discussed four different coordinate systems: $${\bf r} = (r, \theta, \phi).$$ This sysytem can be used in three dimensions. It is particularly suitable for systems with spherical symmetry or functions given in terms of these coordinates. + Infinitesimal distance: $$ds^2 =r^2 (\sin^2 \theta d\phi^2 + d\theta^2) + dr^2 .$$ Infinitesimal volume: @@ -402,7 +405,8 @@ We have discussed four different coordinate systems: 5. $r=1$ and $\theta=\pi/4$ in spherical coordinates, 6. $\varphi=\pi/2$ and $\theta=\pi/2$ in spherical coordinates. -3. [:smirk:] +3. [:smirk:] *Partial derivatives* + (a) Consider the function $f(r,\varphi,\theta)=\frac{1}{r^2}$ defined using spherical coordinates. Compute $\frac{\partial}{\partial z} f(r, \varphi, \theta)$. @@ -412,14 +416,18 @@ We have discussed four different coordinate systems: question). Compute again $\frac{\partial}{\partial z} f(r, \varphi, z)$. -4. [:smirk:] From the transformation from polar to Cartesian +4. [:smirk:] *Chain rule practice* + + From the transformation from polar to Cartesian coordinates, show that $$\frac{\partial}{\partial x} = \cos\varphi \frac{\partial}{\partial r} - \frac{\sin\varphi}{r} \frac{\partial}{\partial \varphi}$$ and $$\frac{\partial}{\partial y} = \sin\varphi \frac{\partial}{\partial r} + \frac{\cos\varphi}{r} \frac{\partial}{\partial \varphi}.$$ (Use the chain rule for differentiation). -5. [:sweat:] Using the result of problem 4, show that the Laplace +5. [:sweat:] *Laplace operator in spherical coordinates* + + Using the result of problem 4, show that the Laplace operator acting on a function $\psi({\bf r})$ in polar coordinates takes the form $$\nabla^2 \psi({\bf r}) =\left( \frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2}\right) \psi({\bf r}) = \frac{1}{r} \frac{\partial}{\partial r} \left( r \frac{\partial \psi(r,\varphi)}{\partial r} \right) + \frac{1}{r^2} \frac{\partial^2 \psi(r,\varphi)}{\partial \varphi^2}.$$ @@ -433,6 +441,7 @@ We have discussed four different coordinate systems: This is however even more tedious (you do not have to show this). 6. [:grinning:] *Integration and coordinates I* + We define $f(r, \varphi) = \frac{1}{r}$ in polar coordinates. Explain how a circular region, centered at the origin and with radius $r_0$, can be described using polar coordinates. Then compute the integral of $f(r,\varphi)$ over @@ -444,7 +453,7 @@ We have discussed four different coordinate systems: 8. [:smirk:] *Integration and coordinates III* - In 2D we can define a shape by specifying a function $r(\varphi)$: + In 2D, we can define a shape by specifying a function $r(\varphi)$: 