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Ohm's law

Ohm's law states that at constant temperature, the current through a conductor is directly proportional to the potential difference across it.

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Student-friendly explanation

For a given conductor kept at the same temperature, increasing voltage increases current in the same proportion. This relation is one of the most important ideas in circuit numericals and graphs. It is written as V = IR.

How to write this in exams

  1. 1

    Start with the exact idea

    Ohm's law states that at constant temperature, the current through a conductor is directly proportional to the potential difference across it.

  2. 2

    Then show how to use it

    1. Check that temperature is constant. 2. Write V = IR. 3. Substitute known values. 4. Rearrange the formula if needed. 5. Calculate and write the unit.

  3. 3

    Add one concrete example

    If a resistor has 2 A current at 6 V, then its resistance is 3 ohm.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is to write R = I/V or to use the law even when the conductor is heating up. That breaks the condition of use.

Definition

Ohm's law states that at constant temperature, the current through a conductor is directly proportional to the potential difference across it.

Example

If a resistor has 2 A current at 6 V, then its resistance is 3 ohm.

Rule to remember

V = IR, so R = V/I and I = V/R. The law applies for a conductor at constant temperature.

Memory hook

V pushes, I flows, R resists.

Examples and method

Worked example

If V = 8 V and I = 2 A for a resistor at constant temperature, then R = V/I = 8/2 = 4 ohm.

Method to apply

1. Check that temperature is constant. 2. Write V = IR. 3. Substitute known values. 4. Rearrange the formula if needed. 5. Calculate and write the unit.

Diagram support

Use a simple circuit with a resistor, ammeter in series, voltmeter across the resistor, and a straight-line V-I graph if needed.

How CBSE asks it

Commonly asked as a numerical, a V-I graph question, or a small reasoning question about proportional change.

Avoid common mistakes

Common confusion

Students sometimes use the formula without keeping temperature constant or swap the quantities wrongly.

Common wrong answer

A common wrong answer is to write R = I/V or to use the law even when the conductor is heating up. That breaks the condition of use.

Exam tip

Always check that the question says constant temperature before using the law directly.

Quick check

A resistor carries 2 A when 6 V is applied. If temperature stays constant and voltage becomes 9 V, what current flows?

The current becomes 3 A because V and I are directly proportional, so 9 V is one and a half times 6 V, and the current also becomes one and a half times 2 A.

Study the ohm's law diagram carefully

Use the labelled diagram to keep ohm's law clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of ohm's law in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on ohm's law.

Revision cue

Revise ohm's law through the labels before writing the answer.

Answer writing and exam use

1-mark answer

Ohm's law states that at constant temperature, the current through a conductor is directly proportional to the potential difference across it.

2-mark answer

Ohm's law states that at constant temperature, the current through a conductor is directly proportional to the potential difference across it. V = IR, so R = V/I and I = V/R. The law applies for a conductor at constant temperature. If a resistor has 2 A current at 6 V, then its resistance is 3 ohm.

3-mark answer

For a given conductor kept at the same temperature, increasing voltage increases current in the same proportion. This relation is one of the most important ideas in circuit numericals and graphs. It is written as V = IR. V = IR, so R = V/I and I = V/R. The law applies for a conductor at constant temperature. If V = 8 V and I = 2 A for a resistor at constant temperature, then R = V/I = 8/2 = 4 ohm. Commonly asked as a numerical, a V-I graph question, or a small reasoning question about proportional change. A common wrong answer is to write R = I/V or to use the law even when the conductor is heating up. That breaks the condition of use.
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