Class 10 > Unit # 15: Current Electricity > Ohm's Law

Talha's Physics Academy

Ohm's Law and Its Limitations

Q.3 State and Explain Ohm’s Law. Also write limitations of Ohm’s Law.

Statement of Ohm's Law

"The current passing through a conductor is directly proportional to the potential difference applied across the ends of the conductor, provided that the physical conditions (such as temperature, mechanical strain, and physical state) of the conductor remain constant."

Mathematical Explanation & Derivation

If $I$ represents the electric current passing through a conductor and $V$ represents the potential difference between its ends, then according to Ohm's Law:

$I \propto V \quad \text{or} \quad V \propto I$

Writing it as an equation:

$I = K V \quad \text{--- (i)}$

Where $K$ is a constant known as the conductance of the material of the conductor. Its value varies for different materials. The reciprocal of conductance is called electric resistance ($R$):

$R = \frac{1}{K} \quad \text{or} \quad K = \frac{1}{R}$

Substituting $K$ into equation (i):

$I = \frac{1}{R} V \quad \implies \quad V = I R$

This fundamental relation is known as Ohm's Law equation.

Fig: Linear V-I characteristic graph for an ohmic conductor obeying Ohm's Law.

Limitations of Ohm's Law

Ohm's Law is an empirical law and does not hold universally under all conditions. The key limitations include:

1. Non-Ohmic Behavior under Electric Fields:

   Some materials and electronic devices (such as semiconductors, thermistors, and diodes) do not follow Ohm's law even under weak electric fields; their $V-I$ relationship is non-linear.

2. Temperature Dependence:

   Ohm's law holds true only at a constant temperature because the electrical resistivity of metals changes with temperature. As current continues to flow through a conductor, Joule heating raises its temperature, altering its resistance ($R$).

3. Joule Heating Effects:

   Continuous current flow produces internal heat within a conductor, governed by Joule's Law of heating ($H = I^2Rt$), which changes the physical and electrical parameters of the system.

4. Complex Network Circuits:

   Ohm's law cannot be directly applied to complex interconnected network circuits without using advanced analytical techniques like Kirchhoff's Laws.

5. Reactive Circuits (Capacitors and Inductors):

   Ohm's law does not apply directly to alternating current (AC) circuits containing pure capacitors or inductors, where impedance, phase shifts, and frequency-dependent reactance govern the circuit behavior.

6. Non-Linear V-I Characteristics:

   For non-ohmic conductors (e.g., semiconductor diodes), the $V-I$ graph is not a straight line and exhibits pronounced variation, making a constant resistance value inapplicable.

© 2026 Talha's Physics Academy. All rights reserved.