8.2.28. Without delving into the question of the origin of external forces, plot the potential of an open and closed circuit with resistance $R$. On a section of the circuit of length $l$, the force assigned to the unit of charge is equal to $E_c$, and outside this section it is zero. What energy per unit charge is transmitted by the source of external forces in the section $l$?
Solution
For problem $8.2.28$
The force per unit charge $E_c$ is equivalent to the electric field strength. Thus, the work done per unit charge by the force on a section $l$ of the circuit is $W=E_c\,l$, which is the same as the potential difference between points $C$ and $B'$. The graphs show potential difference between each point in the circuit and point $A$. When the circuit is open (shown in red), the only change in potential is across the section of length $l$. When the circuit is closed (shown in green), there is also potential drop of $-E_c\,l$ across the resistor. (Note the discontinuity in the potential between points $C'$ and $A$ in the open circuit.)
