Final answer:
The magnitude units of an electric potential contour plot can be understood by relating the potential difference to distance, which results in electric field units of V/m. Equipotential lines signify constant electric potential, and the field strength can be calculated as potential difference over distance.
Step-by-step explanation:
To understand the magnitude units of an electric potential contour plot, it is important to recognize the relation between electric potential and electric field strength. In a contour plot, equipotential lines indicate areas of constant electric potential. The magnitude of the electric field can be understood as the potential difference divided by the distance, which has units of volts per meter (V/m). Using this relationship when a potential isoline graph has isolines every 5.0 V, and six lines cross a 40 cm path, the magnitude of the average electric field along this path can easily be calculated by dividing the total potential difference (30 V) by the distance (0.4 m), which gives 75 V/m.