Electricity | Class 10 Science Notes

11.4 OHM'S LAW

Ohm's law establishes the fundamental relationship between the potential difference across a conductor and the current flowing through it. To explore this, an activity is conducted using a circuit with a nichrome wire, an ammeter, a voltmeter, and cells.

By varying the number of cells (and thus the potential difference), the current through the wire is measured. The ratio of potential difference (V) to current (I) remains approximately constant, indicating a linear relationship.

Mathematically, Ohm's law states that the potential difference V across a conductor is directly proportional to the current I flowing through it, provided the temperature remains constant. This can be expressed as V ∝ I or V / I = constant = R, where R is the resistance of the conductor.

Resistance is a measure of how much a conductor opposes the flow of current. Its SI unit is ohm (Ω). If a conductor has a potential difference of 1 V across it and a current of 1 A flows, its resistance is 1 Ω.

Ohm's law is written as V = IR, and rearranged as I = V / R. The current through a resistor is inversely proportional to its resistance.

Variable resistance devices, such as rheostats, are used to regulate current in circuits without changing the voltage source.

This section introduces the key concept of resistance and the quantitative relationship between voltage, current, and resistance, which is foundational for circuit analysis.

📊 Diagram: Figure 11.2 Electric circuit for studying Ohm's law; Figure 11.3 V-I graph for a nichrome wire. A straight line plot shows that as the current through a wire increases, the potential difference across the wire increases linearly – this is Ohm's law.

🧪 Activity: Activity 11.1: Set up a circuit with nichrome wire, ammeter, voltmeter, and cells; vary number of cells; measure current and voltage; calculate V/I; plot graph; verify Ohm's law.

🔗 Connection: This section leads to understanding the factors affecting resistance of a conductor.

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