# Ohm’s Law

If the physical conditions of the conductor (length, temperature, mechanical strain, etc.) remains same, then the current flowing through the conductor is directly proportional to the potential difference across its two ends, i.e., i V  V = iR, where R is the proportionality constant, known as electric resistance.
• Ohm’s law is not a universal law. The substances which obey Ohm’s law are known as ohmic substances.
• Graph between V and i for a metallic conductor is a straight line as shown in Fig. 9. At different temperatures, V–i curves are different.
Fig. 9
• The devices or substances which do not obey Ohm’s law, e.g., gases, crystal rectifiers, thermoionic valve, transistors, etc., are known as non-ohmic or non-linear conductors. For these V–i curve is not linear.

Fig. 10

Static resistance,

Dynamic resistance,