Physics · Electrostatic Potential And Capacitance · NEET
A polar molecule has a permanent electric dipole moment even with no external field, because its positive and negative charge centres do not coincide (example: H2O, HCl). A non-polar molecule has zero dipole moment normally because the charge centres overlap, but it gains an induced dipole when a field is applied (example: CO2, CH4). Memory hook: "Polar = Permanent, Non-polar = None (until a field pushes it)."
Polar molecule: positive and negative charge centres are separated, giving a permanent dipole moment p even without a field. Non-polar molecule: centres coincide so p = 0, and a dipole appears only when an external field is applied.
Your doubts, answered
What exactly makes a molecule polar?
In a polar molecule the centre of positive charge and the centre of negative charge do NOT lie at the same point. This built-in separation gives it a permanent electric dipole moment p, which exists even when no external field is present. Water (H2O) and HCl are standard NEET examples because their shape is asymmetric.
Why is CO2 non-polar even though it has polar C=O bonds?
Each C=O bond is polar, but CO2 is a straight (linear) symmetric molecule O=C=O. The two bond dipoles point in exactly opposite directions and cancel, so the centres of positive and negative charge end up at the same place. Net dipole moment = 0, so CO2 is non-polar. NCERT lists CO2 and CH4 as non-polar.
Do non-polar molecules stay non-polar in a capacitor?
No. When you place a non-polar dielectric in an external field E, the positive and negative charges are pushed in opposite directions until the internal restoring force balances them. This creates an INDUCED dipole moment in the direction of the field. So a non-polar molecule becomes a temporary dipole only while the field is on; remove the field and it returns to zero dipole.
How do polar molecules respond to a field differently from non-polar ones?
Polar molecules already have a permanent dipole. In a field they mainly ROTATE (align) so their dipoles point along E, though thermal motion resists this. Non-polar molecules have no dipole to rotate; the field must first CREATE (induce) a dipole by stretching the charge separation. Both effects reduce the net field inside the dielectric, which is why any dielectric raises capacitance.
Is dipole moment a vector, and which way does it point?
Yes, dipole moment p is a vector. By convention it points from the negative charge to the positive charge, and its magnitude is p = q x d (charge times separation). For NEET, remember p is measured in coulomb-metre (C m); a typical molecular value is about 10^-29 C m.
⚠️ The NEET trap ✗ Choosing that polar molecules acquire a dipole moment only when a field is applied. ✓ Polar molecules have a PERMANENT dipole moment even with NO external field. It is the non-polar molecule that gains a dipole (induced) only when a field is applied. 🧠 Permanent = polar (no field needed); Induced = non-polar (field needed). Do not swap them.
Real NEET questions
2021
Polar molecules are the molecules:
A · Acquire a dipole moment only when magnetic field is absent
B · Having a permanent electric dipole moment ✓
C · Having zero dipole moment
D · Presence of electric field due to displacement of charges
Solution: Step 1: Identify the defining property. A polar molecule has its centres of positive and negative charge separated, so it carries a dipole moment on its own. Step 2: Check if a field is needed. This separation is built into the shape (for example H2O, HCl), so the dipole exists even in the absence of any external electric field — it is PERMANENT. Step 3: Reject the wrong options. Zero dipole moment describes non-polar molecules (option C). A dipole formed only because a field displaces charges (option D) describes an INDUCED dipole in a non-polar molecule, not a polar one. Magnetic field (option A) is irrelevant to electric polarity. Correct answer: (B) having a permanent electric dipole moment.
Solved Electrostatic Potential And Capacitance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Give NEET examples of polar and non-polar molecules.
Polar (permanent dipole): H2O (water), HCl. Non-polar (zero dipole): CO2, CH4. These are the exact examples used in NCERT Physics.
What is an induced dipole moment?
It is the dipole moment created in a molecule by an external electric field, which pulls positive and negative charges slightly apart. Non-polar molecules rely entirely on this; it disappears once the field is removed.
Why does any dielectric increase capacitance regardless of polar or non-polar?
In both cases the field polarises the dielectric and produces bound charges whose field opposes the applied field. This lowers the net field and the voltage between the plates, so C = Q/V increases from C0 (vacuum) to K x C0.
Are all molecules with polar bonds polar molecules?
No. A molecule can have polar bonds yet be non-polar overall if its symmetry makes the bond dipoles cancel, as in CO2 and CH4. Net dipole moment, not individual bonds, decides polarity.
What is the unit of dipole moment?
The SI unit is coulomb-metre (C m). Molecular permanent dipole moments are of the order of 10^-29 C m.
