Ohm’s law states that the current density \( \overrightarrow{j} \) is proportional to the electric field \( \overrightarrow{E} \), with the conductivity \( \sigma \) as the constant of proportionality: \[ \overrightarrow{j} = \sigma \overrightarrow{E} \] This expresses the relationship between the electric field and current density inside a conductor.
A battery of emf \( E \) and internal resistance \( r \) is connected to a rheostat. When a current of 2A is drawn from the battery, the potential difference across the rheostat is 5V. The potential difference becomes 4V when a current of 4A is drawn from the battery. Calculate the value of \( E \) and \( r \).