What is Ohm’s Law? Explanation from CBSE Class 12 Physics Syllabus

Ohm’s law is an important concept to learn from the chapter on electricity in Physics. Here the law has been explained in detail for your understanding to feel confident solving questions regarding this topic. Laws like such are important from the JEE Main 2023 point of view. If you find physics a tough nut to crack, Find Aakash Coaching Institute Near You so that you can attend face-to-face classes.

Ohm’s Law

Ohm’s Law is named after the German physicist George Ohm, as he was the first person to prove this law experimentally. 

George Simon Ohm established the relation between electric current and potential difference. Ohm’s law states, “Potential applied across the end of the conductor is directly proportional to the current flowing through a conductor, provided the physical conditions (such as temperature) remain unchanged.’’

                                           V    =    IR or I  =  V/R,

where R is the constant of proportionality called resistance of the circuit

I in the current in amperes

V is the voltage across the conductor in volts.

From the above formula, it is clear that current is inversely proportional to resistance. If resistance is doubled, the current is halved, and if the resistance is halved, the current is doubled. If understanding this law is tricky for you, check NCERT Solutions for Class 12 Physics and NCERT Solutions. It will help you a lot. Check NCERT Solutions for Class 12 if you want to learn other subjects. Ohm’s Law is a reliable relation that describes the conductivity of many electrically conductive materials. However, some materials do not follow Ohm’s Law. Those are called non-ohmic. Examples are diodes.

An Experimental Analogy to Understand the Concept of Ohm’s Law

• Let us take a beaker filled with water.
• Let us make three holes in the beaker vertically and attach three water pipes to the holes so that water flows through them from the beaker to the outside.
• We find that the water flowing through the uppermost water pipe is high, the flow of water through the middle water pipe is relatively lower, and the speed of the water flowing through the lowermost water pipe is the lowest.
• Also, we notice that the uppermost water pipe is lying closest to the beaker; the middle one is a little farther away, while the lowermost water pipe lies farthest away from the beaker.
• The water adjacent to the uppermost pipe is the uppermost layer, so its pressure is low. The water adjacent to the middle water pipe has one layer above it so its pressure is higher. The water adjacent to the lowermost water pipe has two layers of water, so the pressure is the highest. So, the pressure in the three water layers is different. This pressure difference resulted in a speed difference.
• Water always travels from a higher concentration to a lower concentration, that is, against a concentration gradient. That is why the water moves from inside to outside the beaker. The pressure of water outside the beaker is 0 Pascal.
• Suppose the water pressure in layer one is 5 Pascal, the pressure of water in the next layer is 10 Pascal, and the pressure of water in the third layer is 15 Pascal.
• Let’s compare the pressure difference of the first layer of water with the pressure outside the beaker. It will come out to be equal to 5 Pa (5-0), the pressure difference for the next layer comes out to be 10 Pa (10-0), and the pressure difference for the lowermost layer comes out to be 15 Pa (15-0).
• From this discussion, we conclude that the more the pressure difference, the more the speed of water flow.
• The same rule is followed by airflow. It always moves from a high-pressure gradient to a lower-pressure gradient. If the pressure difference between the two points through which airflow is high, the speed of air increases.
• To understand Ohm’s Law through this analogy, let us substitute the flow of water from the beaker with the flow of current, the pressure difference with the potential difference. So we can say that the more the potential difference, the more the flow of current. The more the difference in the voltage of two conductors, the more the speed of electric current flows through them. So, voltage is directly proportional to current and inversely proportional to resistance. More the resistance, the slower the current flows.

Ohmic and Non-Ohmic Conductors

Based on whether or not conductors follow Ohm’s Law, they can be divided into two types:

• Ohmic conductors
• Non-ohmic conductors
• Ohmic conductors: Those conductors that follow Ohm’s Law, provided the physical conditions such as temperature remain constant are called ohmic conductors. A perfect example of an ohmic conductor is a transistor. Some more examples are metals like silver, copper, and aluminum. The VI graph of ohmic conductors is linear. 
• Non-ohmic conductors: These conductors have their unique characteristics, due to which they do not follow Ohm’s Law. Examples of non-ohmic conductors are diodes. Graphically speaking, the slope of the current and voltage is a curved line for non-ohmic conductors, unlike ohmic conductors, where it is straight.

Limitations of Ohm’s Law

1. It does not apply to non-ohmic circuits.
2. Even inside ohmic circuits, there are some limitations of this law. For example, suppose a large current flows through a conductor, according to H=I²RT. In that case, heating takes place due to which the resistance of the conductor also increases and because of high resistance, instead of getting a straight line, we get a curve. 
3. The temperature and physical state of the material has to remain constant to fulfil Ohm’s Law.

Therefore, Ohm’s Law is not universal.

VI Graph of Ohm’s Law

• The graph between the potential difference V and the corresponding current I is a straight line, passing through the origin for ohmic (metal) conduction.
• The current I is directly proportional to the potential difference V and inversely proportional to the resistance. The current flows through a conductor slower if the resistance is high.

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Conclusion

If you are studying Physics for Class 12th, you will likely be planning to compete in the JEE Main examination shortly. If you start preparing for competitive exams at the right time, you will have a headstart and better qualify for them. Get yourself acquainted with the JEE Main 2023 Exam pattern and JEE Main 2023 Syllabus by checking JEE Main Exam Pattern and JEE Main Syllabus to stay ahead of the game.  

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