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Challenging3 past questions

Drift Velocity and Microscopic View of Current

Explore the microscopic origin of current, including drift velocity, relaxation time, current density, and the effect of electric field in conductors.

Why this shows up in the exam

NEET often asks about the microscopic explanation of current and related calculations.

How NEET tests this

Statement analysis · 2 QsMatch the columns

Learn the idea

Microscopic current arises because an electric field inside a conductor makes charge carriers acquire a small average drift velocity; the faster the field, the larger the drift speed.

🧠 Memory hook: A gentle wind (E) pushes a crowd of walkers; the longer they glide between bumps (τ), the farther they drift – v_d = wind × glide‑time.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Current density J = n e v_d
  • Drift velocity v_d = e E τ / m (τ = mean free time)
  • Conductivity σ = n e² τ / m and J = σ E
  • A steady current exists only if a non‑zero electric field is present inside the conductor
  • Electrons move opposite to the direction of the electric field, so conventional current is opposite to electron motion

How to approach it

  1. 1Read the statement and decide whether it mentions an electric field inside the conductor
  2. 2If current is claimed, verify with J = σ E (or J = n e v_d) that a field is required
  3. 3Use v_d = e E τ / m (or v_d = I/(n e A)) to see how drift speed changes with the field
  4. 4Choose true/false and explanation based on the proportionality of v_d with E

Worked example — watch it click

Statement-I: A current flows in a conductor only when there is an electric field within the conductor. Statement-II: The drift velocity of electrons in the presence of an electric field decreases.

  • A)Statement I is true, Statement II is true, and Statement II is the correct explanation of Statement I.
  • B)Statement I is true, Statement II is true, but Statement II is not the correct explanation of Statement I.
  • ✅Statement I is true, Statement II is false.
  • D)Statement I is false, Statement II is true.

The concept behind this problem

The example asks you to recognise that a conductor needs an internal electric field for current and that the drift velocity grows with that field, not shrinks.

Step by step

  1. 1Statement I is true: current requires electric field (J = σE).
  2. 2Statement II is false: drift velocity increases in presence of electric field (v_d = eEτ/m).

Watch out

Students often mark Statement II true, assuming collisions slow the electrons, but v_d ∝ E, so the statement is false.

Common slip-ups that cost marks

  • •Mixing up the direction of electron motion with conventional current direction
  • •Thinking the drift speed falls when the field is applied – it actually rises proportionally with E
  • •Confusing τ (mean free time) with the drift speed itself

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 3

Assertion (A): A wire carrying an electric current has no electric field around it. Reason (R): Rate of flow of electrons in one direction is equal to the rate of flow of protons in the opposite direction.

Push further

More challenging

8 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 8

Which of the following statements correctly describes the relationship between current density (J), drift velocity (vd), and electric field (E) in a metallic conductor?

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