Assumptions and Limitations of Ohm's Law

Ohm's law states that the current flowing through a conductor is directly proportional to the voltage across it. On the other hand, the current flowing is inversly proportional to it resistance when the temperature remains constant. Mathematically, it is expressed as:

V = IR

which also translates to:

I = V/R

R= V/I

NCERT exercise on the current electricity chapter provides practice questions related to Ohm’s law for students.

For CUET exam and IISER exam aspirants, let us take a look at some of the assumptions of Ohm's law:

1. Constant Temperature: As per Ohm's law, it is assumed that the temperature of the conductor will remain constant. However, in reality, the resistance of materials changes with temperature, thus affecting the relationship between voltage and electric current.
2. Linear Relationship: Ohm's law also assumes that there is a linear relationship between voltage and current. This shows that resistance R is constant and it does not change even if the voltage or current is applied.
3. Homogeneous Material: It is also assumed that the conductor is homogeneous, thus, it will have uniform properties. It also means that, as per the assumption, the resistivity is uniform and the physical dimensions including length and cross-sectional area are consistent.
4. No external Influences: Ohm's law also makes the assumption that no external factors are affecting the conductor which could have otherwise changed the elctrical properties of the material.
5. Isotropic Conductivity: As per the law, it is assumed that in all the directions, the electrical properties of the material will remain the same. However, this is not always the case. 

Let us understand the limitations of Ohm’s law, which are important for JEE Main entrance exam and IIT JAM exam point of view: 

• Metallic conductors can act like ohmic substance to a certain current level but later on, at larger currents; they become non-ohmic.

• Ohm's law limitations only applies to ohmic materials in which there is a linear relationship between voltage and electric current . Any other material which does not show this linear relationship such as semiconductors, vacuum tubes, and gas discharge tubes are not applicable under Ohm's Law.
• As the frequency becomes higher, inductance and capacitance effects become very impactful. At such frequencies, Ohm's Law is insufficient for AC circuit analysis.
• Ohm's law is not applicable to those devices that have p-n junctions like diodes, transistors, and LEDs. This happens because these devices show non-linear voltage-current characteristics.
• At higher power levels, materials can change their properties because of heating, breakdown, and other effects. In these cases, the ohm's law statement is not applicable.
• In stronger magnetic fields, Hall effect can change the relationship between voltage, current, and resistance. Due to this ohm’s law fail.
• After a particular voltage threshold, materials undergo breakdown, which invalidates the definition of Ohm's Law.