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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.

In one line: V pushes, I flows, R resists.

Practice This Concept

Understand it

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.

Example

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

Formula and rule

V = IR, so R = V/I and I = V/R.

The law applies for a conductor at constant temperature.

See it worked out

Follow the example line by line, then use the same method on new questions.

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 you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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.

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.

Don't mix this up

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

Exam tip

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

Ohm's law diagram

Open full screen to read every label clearly.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

1
Enough for 1 mark
Ohm's law states that at constant temperature, the current through a conductor is directly proportional to the potential difference across it.
2
Add for 2 marks
Example: If a resistor has 2 A current at 6 V, then its resistance is 3 ohm.
3
Add for 3 marks
For a given conductor kept at the same temperature, increasing voltage increases current in the same proportion.

MCQ Quiz

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