This law gives relationship between the potential difference (V), the current (I) and the resistance (R) of a d.c. circuit. Dr. Ohm in 1827 discovered a law called Ohm's law. It states,

Ohm's law : The current flowing through the electric circuit is directly proportional to the potential difference across the circuit and inversely proportional to the resistance of the circuit, provide the temperature remains constant.

Where I is the current flowing in amperes, the V is voltage applied and R is the resistance of the conductor, as shown in the fig 1.

Fig. 1 Ohm's law |

The unit of potential difference is defined in such a way such the constant of proportionality is unity.

**The Ohm's law can be defined as,**

The ratio of potential difference (V) between any tow points of a conductor to the current (I) flowing between them is constant, provided that the temperature of the conductor remains constant.

**Key point**: Ohm's law can be applied either to the entire circuit or to the part of a circuit. If it is applied to entire circuit, the voltage across the entire circuit and resistance of the entire circuit should be taken into account. If the Ohm's law is applied to the part of a circuit, then the resistance of that part and potential across that part should be used.

**Limitations of Ohm's law**

1) It is not applicable to the non linear devices such as diodes, zener diodes, voltage regulators etc.

2) It does not hold good for non-metallic conductors such as silicon carbide. The law for such conductors is given by,

V = K I

^{m}Where K, m are constants.
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