Mathematically, Ohm’s Law is written: V = IR
V = potential difference [V]
I = current [A]
R = resistance [Ω]
Ohm’s Law tells us that if a conductor is at a constant temperature, the voltage across the
ends of the conductor is proportional to the current. This means that if we plot voltage on the
y-axis of a graph and current on the x-axis of the graph, we will get a straight-line. The
gradient of the straight-line graph is then the resistance of the conductor.
The resistance, R is a term that describes the opposition experienced by the electrons as they flow in a conductor. It is also defined as the ratio of the potential difference across the conductor to the current, I flowing through the conductor.
Non-ohmic conductors are conductors which do not obey Ohm’s Law.
E.g., a light bulb whose resistance increases over time due to temperature increase (heating effect of current)
In general non-ohmic conductors have plots of voltage against current that are curved,
indicating that the resistance is not constant over all values of voltage and current.
Worked example :
What is the value of the resistor in the figure, if the dry cells supply 2.0 V and the ammeter reading is 0.5 A?
V = IR
2.0 = 0.5 (R)
R = 4 Ω