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Exam level3 past questions

Coulomb's law and electric field of point charges

Understand Coulomb's law, the principle of superposition, and how to calculate the electric field due to point charges and simple charge distributions.

Why this shows up in the exam

NEET tests your ability to apply Coulomb's law and calculate electric fields in various configurations.

How NEET tests this

Numerical · 2 QsAssertion–reasonStatement analysis

Learn the idea

Coulomb's law gives the force between two point charges and the electric field of a single point charge follows the same inverse‑square dependence. The key insight is that the field is a property of the source charge alone – you just plug E = kq/r² and then use vector superposition for any configuration.

🧠 Memory hook: Coulomb’s law is the electric twin of gravity: both are inverse‑square, but replace mass with charge – think “C for Charge, C for inverse‑Square”.

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

Formulas & facts to keep ready

  • Coulomb's law: F = (1/4πϵ₀)·(q₁q₂/r²) along the line joining the charges
  • Electric field of a point charge: E = (1/4πϵ₀)·(q/r²) radially outward for q>0, inward for q<0
  • Principle of superposition: net electric field = vector sum of fields due to individual charges
  • Electrostatic field is conservative: work done moving a test charge between two points is path‑independent
  • Acceleration in an electric field: a = qE/m, independent of the particle’s velocity

How to approach it

  1. 1Read the question and decide whether it asks for force, field, work or acceleration
  2. 2Write the appropriate formula from the key ideas (Coulomb’s law or E = kq/r²) and note the sign/direction
  3. 3If more than one charge is involved, draw vectors and add them head‑to‑tail (superposition)
  4. 4Insert given numerical values, keep the 1/r² dependence, and solve for the required quantity

Worked example — watch it click

Assertion A: Acceleration of a charged particle in a non-uniform electric field does not depend on the velocity of the charged particle. Reason R: Charge is an invariant quantity. The amount of charge on a particle does not depend on the frame of reference.

  • A)If both assertion and reason are true, and reason is the correct explanation of assertion.
  • ✅If both assertion and reason are true, but reason is not the correct explanation of assertion.
  • C)If assertion is true but reason is false.
  • D)If both assertion and reason are false.

The concept behind this problem

The worked example probes whether you know that a charged particle’s acceleration in an electric field comes from F = qE, which contains no velocity term, and that charge being invariant is a true statement but not the causal reason for the velocity‑independence.

Step by step

  1. 1Assertion A: For a charged particle in an electric field, F = qE, so acceleration a = qE/m.
  2. 2In a non-uniform field, E varies with position but not with the velocity of the particle.
  3. 3The acceleration depends only on q, E (at that position), and m - not on velocity v.
  4. 4Therefore, Assertion A is TRUE.
  5. 5Reason R: Charge is indeed an invariant quantity - it does not change with frame of reference (unlike mass in relativistic mechanics).
  6. 6This statement is TRUE.
  7. 7However, the reason (charge being invariant) is not the direct explanation for why acceleration doesn't depend on velocity.
  8. 8The real reason is that electric force F = qE is independent of velocity (unlike magnetic force F = qv×B).
  9. 9While R is a true statement about charge, it doesn't explain why acceleration is velocity-independent in an electric field.
  10. 10Therefore, both A and R are true, but R is not the correct explanation of A.

Watch out

Students often claim that because charge is invariant, acceleration must be velocity‑independent, confusing a true fact with the actual explanation.

Common slip-ups that cost marks

  • •Mixing up force and field – force needs the test charge q, field does not
  • •Ignoring the sign of the source charge when assigning direction of E
  • •Assuming acceleration depends on velocity because the field is non‑uniform; electric force itself has no velocity term

🌟 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

Statement-I: The electric field due to a point charge decreases as the distance from the charge increases. Statement-II: The work done in moving a test charge from one point to another in an electric field is independent of the path taken between the points.

Push further

More challenging

9 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 9

A charged particle with charge q and mass m is moving in a region where both a uniform electric field E and a uniform magnetic field B are present. If the particle's velocity is perpendicular to both E and B, which of the following statements is true about its acceleration?

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