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Exam level6 past questions

Electric dipoles and molecular dipole moments

Study the properties of electric dipoles, their fields and potentials, behavior in electric fields, and the distinction between polar and non-polar molecules.

Why this shows up in the exam

You need to analyze dipole behavior and field interactions, which are common in NEET questions.

How NEET tests this

Numerical · 4 QsDirect recall · 3 QsStatement analysis · 2 Qs

Learn the idea

A dipole consists of two equal opposite charges separated by a small distance; its strength is the vector p = q · d pointing from –q to +q. In the far‑field (r≫d) the dipole’s field and potential fall as 1/r³ and 1/r², and the sign (direction) depends on whether you are on the axial line or the equatorial plane.

🧠 Memory hook: Think of a dipole as a tiny barbell: push forward twice as hard on the tip (axial, +2), but push back once on the side (equatorial, –1).

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

Formulas & facts to keep ready

  • Dipole moment p = q d (vector from –q to +q)
  • Potential V = (1/4π ε₀) (p·r̂)/r² for r≫d
  • Axial field E = (1/4π ε₀) (2p)/r³, direction same as p
  • Equatorial field E = –(1/4π ε₀) p/r³, direction opposite to p
  • Torque τ = p × E and potential energy U = –p·E
  • Polar molecule: net permanent dipole moment; non‑polar: zero net dipole moment

How to approach it

  1. 1Identify whether the point is on the axial line or the equatorial plane of the dipole
  2. 2Write the appropriate far‑field formula (use 2p/r³ for axial, –p/r³ for equatorial)
  3. 3Insert the given magnitude of p and distance r, keep track of the sign to get direction
  4. 4If the question asks for potential, use V = (1/4π ε₀)(p·r̂)/r² and evaluate the dot product

Worked example — watch it click

The electric field at a point on the equatorial plane at a distance r from the centre of a dipole having dipole moment ⃗p is given by (r >> separation of two charges forming the dipole, (ε₀ = permittivity of free space)

  • A)⃗E = ⃗p / 4πε₀r³
  • B)⃗E = 2⃗p / 4πε₀r³
  • C)⃗E = −⃗p / 4πε₀r²
  • ✅⃗E = −⃗p / 4πε₀r³

The concept behind this problem

The example checks that you know the equatorial‑plane field is opposite to the dipole moment and varies as 1/r³, a hallmark of dipole behaviour far from the source.

Step by step

  1. 1On the equatorial plane of a dipole, at distance r >> separation, the electric field magnitude is E = p/(4πε₀r³), directed opposite to the dipole moment vector p⃗ (from +q to -q).
  2. 2In vector form: E⃗ = -p⃗/(4πε₀r³).
  3. 3The negative sign indicates the field points opposite to p⃗.
  4. 4Option (d) is correct.
  5. 5Note: On the axial line, E⃗ = 2p⃗/(4πε₀r³) (same direction as p⃗).

Watch out

Students often forget the minus sign and write +p/(4π ε₀ r³) instead of the correct –p/(4π ε₀ r³).

Common slip-ups that cost marks

  • •Mixing up the axial factor 2 with the equatorial factor –1
  • •Using r² instead of r³ for the field expression
  • •Dropping the negative sign for the equatorial field, which flips the direction

🌟 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 6NEET 2020

A short electric dipole has a dipole moment of 16 × 10⁻⁹ Cm. The electric potential due to the dipole at a point at a distance of 0.6 m from the centre of the dipole, situated on a line making an angle of 60° with the dipole axis is 1/(4πε₀) = 9 × 10⁹ Nm²/C²

Push further

More challenging

5 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 5

A short electric dipole has a dipole moment of 10 × 10⁻⁹ Cm. What is the electric potential at a point 0.3 m away from the center of the dipole, situated on its equatorial plane? Assume 1/(4πε₀) = 9 × 10⁹ Nm²/C².

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