PhysicsCBSE Class 12 · Board exam 2026, set 55/1/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2026: question 20

Set in the CBSE Class 12 board exam of 2026, paper 55/1/1, section B, question 20, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20 · 2 marks · Very short answerCurrent Electricity

What is the order of magnitude of drift velocity of electrons in a conductor ? Deduce the relation between the current flowing through a conductor and drift velocity of electrons in it.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Think about how far an electron drifts in time $Δ t$ .

Hint 2

Count the electrons in the volume $A v_{d} Δ t$ and multiply by $e$ .

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

The drift velocity of electrons in a conductor is of the order of $1 0^{- 4}$ m/s (about a fraction of a mm/s).
½ mark
Take a conductor of cross-sectional area $A$ with $n$ free electrons per unit volume, each drifting with speed $v_{d}$ . In time $Δ t$ , every electron within a length $v_{d} Δ t$ crosses a given section. The number crossing is $n A v_{d} Δ t$ , so the charge crossing is $Δ Q = n e A v_{d} Δ t$ .
1 mark
Current: $I = \frac{Δ Q}{Δ t} = n e A v_{d}$
½ mark

Answer: $v_{d} \sim 1 0^{- 4}$ m/s; $I = n e A v_{d}$

Where the marks are

States that the order of $v_{d}$ is $1 0^{- 4}$ m/s (½ mark)
Finds the charge crossing a section in time $Δ t$ as $n e A v_{d} Δ t$ (1 mark)
Arrives at $I = n e A v_{d}$ (½ mark)

Revise the chapter

Chapter: Current Electricity (notes, formulas, where marks are lost and every past question)
Topic: Drift Velocity, Mobility and Resistivity

Practise this chapterBoard-style questions from Current Electricity, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 55/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.

CBSE Class 12 Physics 2026: question 20

Hints

Step-by-step solution

Revise the chapter

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