Update 3_drude_model.md - fix

src/3_drude_model.md

@@ -111,8 +111,8 @@ We apply a magnetic field $\bf B$ perpendicular to a current carrying 2D sample.
 
 $$\mathbf{E} = \begin{pmatrix} \rho_{xx} & \rho_{xy} \\ \rho_{yx} & \rho_{yy} \end{pmatrix} \mathbf{J}$$
  
-  1. Sketch $\rho_{xx}$ and $\rho_{xy}$ as a function of the magnetic field $\bf B$. 
-  2. Invert the resistivity matrix to obtain the conductivity matrix $$\begin{pmatrix} \sigma_{xx} & \sigma_{xy} \\ \sigma_{yx} & \sigma_{yy} \end{pmatrix} $$ and express $\mathbf{J}$ as a function of $\mathbf{E}$. 
+  1. Sketch $\rho_{xx}$ and $\rho_{xy}$ as a function of the magnetic field $\bf B$. i
+  2. Invert the resistivity matrix to obtain the conductivity matrix $$\begin{pmatrix} \sigma_{xx} & \sigma_{xy} \\ \sigma_{yx} & \sigma_{yy} \end{pmatrix} $$, allowng you to express $\mathbf{J}$ as a function of $\mathbf{E}$. 
   3. Sketch $\sigma_{xx}$ and $\sigma_{xy}$ as a function of the magnetic field $\bf B$. Calculate the value of $\sigma_{xx}$ for $\rho_{xx}=0$. Discuss what is going on here.
   4. Give the definition of the Hall coefficient. What does the sign of the Hall coefficient indicate?