⚡ Chapter 35 : Ohm's Law (Class XII)
🔷 1. Introduction
Electric current flows through a conductor only when a potential difference (voltage) is applied across its ends. However, the amount of current flowing through the conductor does not depend only on the applied voltage. Every conductor opposes the flow of electric charges to some extent. This opposition is called Resistance.
The relationship among Voltage (V), Current (I), and Resistance (R) was experimentally discovered by the German physicist Georg Simon Ohm in 1827. This relationship is known as Ohm's Law.
🔷 2. Statement of Ohm's Law
Ethan: Professor, what exactly does Ohm's Law state?
Professor: Ohm's Law states that the electric current flowing through a conductor is directly proportional to the potential difference across its ends, provided that the physical conditions such as temperature, dimensions, and material of the conductor remain constant.
Academic Statement
At constant temperature and other physical conditions, the current flowing through a conductor is directly proportional to the potential difference applied across it.
🔷 3. Mathematical Expression
Ethan: Professor, how do we express Ohm's Law mathematically?
Professor: Since current is directly proportional to voltage,
I ∝ V
Removing the proportionality sign by introducing a constant called resistance,
where:
- V = Potential Difference (Volt)
- I = Electric Current (Ampere)
- R = Resistance (Ohm)
🔷 4. Physical Meaning of Ohm's Law
Ethan: Professor, what does this law mean physically?
Professor: Imagine water flowing through a pipe.
- Increasing the water pressure causes more water to flow.
- Narrowing the pipe reduces the flow of water.
Similarly,
- Increasing the voltage increases the flow of electric charges (current).
- Increasing the resistance decreases the current.
Thus, current depends on both the applied voltage and the resistance of the conductor.
🔷 5. Understanding the Formula
Ethan: Professor, how do voltage and resistance affect current?
Professor:
| Condition | Effect on Current |
|---|---|
| Voltage increases | Current increases |
| Voltage decreases | Current decreases |
| Resistance increases | Current decreases |
| Resistance decreases | Current increases |
🔷 6. Conditions for Ohm's Law
Ethan: Does Ohm's Law always work?
Professor: No. Ohm's Law is valid only if certain physical conditions remain unchanged.
- Temperature must remain constant.
- The material of the conductor must not change.
- The length of the conductor must remain constant.
- The cross-sectional area must remain constant.
- The conductor should obey Ohm's Law (Ohmic conductor).
🔷 7. V-I Graph
Ethan: Professor, what does the graph between voltage and current look like?
Professor: For an ohmic conductor, the graph between voltage and current is a straight line passing through the origin.
This indicates that voltage and current are directly proportional.
📦 8. Important Results (Must Remember)
- Current is directly proportional to voltage.
- Current is inversely proportional to resistance.
- Resistance is the proportionality constant.
- Ohm's Law is valid only under constant physical conditions.
- The SI unit of resistance is Ohm (Ω).
- Higher voltage produces higher current.
- Higher resistance produces lower current.
🔷 9. Ohmic and Non-Ohmic Conductors
| Ohmic Conductors | Non-Ohmic Conductors |
|---|---|
| Obey Ohm's Law. | Do not obey Ohm's Law. |
| Straight-line V-I graph. | Curved V-I graph. |
| Example: Metallic wires. | Example: Diodes, filament lamps, thermistors. |
🧠 10. Conceptual Questions
🔹 Q1
Ethan: What does Ohm's Law state?
Professor: Current is directly proportional to voltage if physical conditions remain constant.
🔹 Q2
Ethan: Who discovered Ohm's Law?
Professor: Georg Simon Ohm.
🔹 Q3
Ethan: What is the SI unit of resistance?
Professor: Ohm (Ω).
🔹 Q4
Ethan: Why does current decrease when resistance increases?
Professor: Greater resistance offers more opposition to the movement of charge carriers.
🔹 Q5
Ethan: Why is temperature kept constant in Ohm's Law?
Professor: Because resistance changes with temperature, which would change the relationship between voltage and current.
🔷 11. Applications of Ohm's Law
- Design of electrical circuits.
- Calculation of current, voltage, and resistance.
- Selection of electrical components.
- Household electrical wiring.
- Electronic circuit analysis.
- Power distribution systems.
🔷 12. Summary
Ohm's Law establishes the fundamental relationship between voltage, current, and resistance. It states that, under constant physical conditions, the electric current through a conductor is directly proportional to the applied potential difference and inversely proportional to the resistance. This law forms the foundation of electrical engineering and circuit analysis and is widely used in designing and understanding electrical and electronic systems.
✨ End of Topic: Ohm's Law ✨
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