Current-Potential Difference Graphs

A current-potential difference graph (also known as an I-V graph) shows how the current passing through a component varies with the potential difference. The graph has:

  • Potential difference on the X-axis
  • Current on the Y-axis

Ohmic Conductor

A conductor that obeys Ohm’s law has a constant resistance. The line is straight and passes through the origin. This shows that the current is proportional to the potential difference across the resistor, provided the temperature remains constant.

A graph with two perpendicular axes. The vertical axis is labelled "Current (I)" while the horizontal axis is labelled "Potential difference (V)". A straight red line, representing a linear relationship, diagonally intersects the graph from the origin, indicating a direct proportionality between the current and potential difference.

Non-Ohmic Components

Examples of non-ohmic components are lamps, diodes, light-dependent resistors (LDRs) and thermistors. They do not have constant resistance; the resistance changes as the current passing through the component changes.

Filament bulb

Below is the current-potential difference graph for a filament bulb.

A graph with two perpendicular axes. The vertical axis is labelled "Current (I)" and the horizontal axis is labelled "Potential difference (V)". A red curved line starts from the origin and rises steeply to the right, suggesting a non-linear relationship between the current and potential difference.

The graph for the filament bulb does not obey Ohm’s law because it is not a straight line. The metal filament gets hotter as current flows through it. The metal atoms in the filament vibrate more intensely, which increases the number of collisions with passing electrons. This leads to an increase in resistance, curving the graph.

The reason the graph is symmetrical is that the relationship does not change when the current flows in the opposite direction.

Diodes

Below is the current-potential difference graph for a diode.

A graph with the vertical axis labelled "Current (I)" and the horizontal axis labelled "Potential difference (V)". A red curved line starts from the intersection of the axes and arches upwards to the right, indicating an increasing relationship between current and potential difference as the curve progresses.

As the graph is not a straight line, it does not obey Ohm’s law. Diodes have high resistance in one direction, allowing current to flow only in that direction.