A student measures the current through a device as the potential difference is increased. The resulting I–ΔV graph is a smooth curve that becomes progressively steeper at higher voltages. Which of the following best explains why the device is classified as non-ohmic?
a) Because the graph does not pass through the origin, so the device must contain an internal emf that makes it non-ohmic
b) Because the slope of the graph represents the material’s resistivity, so a changing slope means the resistivity itself is changing with voltage
c) Because its resistance changes with the applied potential difference, violating the condition of constant R
d) Because a curved I–ΔV plot is characteristic of a diode, which only allows current in one direction, therefore the device is non-ohmic