A student connects a cell, of emf ${\epsilon }_2$ and internal resistance $r_2$ with a cell of emf ${\epsilon }_1$ and internal resistance $r_1$, such that their combination has a net internal resistance less than $r_1$. This combination is then connected across a resistance $R$.

Draw a diagram of the 'set-up' and obtain an expression for the current flowing through the resistance $R$.

Plot a graph showing the variation of current density ($j$) versus the electric field ($E$) for two conductors of different materials. What information from this plot regarding the properties of the conducting material, can be obtained which can be used to select suitable materials for use in making standard resistance 

Electron drift speed is estimated to be of the order of $\mathrm{mm} {\mathrm{s}}^{-1}$. Yet a large current of the order of few amperes can be set up in the wire. Explain briefly.

Plot a graph showing the variation of current density ($j$) versus the electric field ($E$) for two conductors of different materials. What information from this plot regarding the properties of the conducting material, can be obtained which can be used to select suitable materials for use in making connecting wires in electric circuits?

Electron drift speed is estimated to be of the order of $\mathrm{mm} {\mathrm{s}}^{-1}$. Yet a large current of the order of few amperes can be set up in the wire. Explain briefly.