EIS observes these processes under steady-state conditions by monitoring the response of the device to a small voltage perturbation. Resistances and capacitances are extracted by modelling the raw data as an equivalent circuit (EC), the components of which represent physical processes occurring within the device. Most processes observed for PEC devices (and all discussed here) appear as semicircles in the Nyquist plot. At potentials where EIS detects only one of these processes, the device is unambiguously modelled with a resistor and capacitor in parallel, known as the Randles circuit. Unlike physical circuit elements, the resistance and capacitance magnitudes may vary over the experimental potential range applied to the device. Moreover, the circuit used to represent the device may vary with the applied potential, as physical processes in the device are often limited to a defined potential range.