What is polarisation of a dielectric? Explain why a dielectric slab placed between the plates of a charged capacitor increases its capacitance.

1. 1 · Define. Polarisation is the lining up of molecular dipoles, induced or permanent, along an applied electric field.
2. 2 · Two kinds. Non-polar molecules (N₂, O₂) get induced dipoles; polar molecules (H₂O, HCl) have permanent dipoles that turn towards the field.
3. 3 · Draw the slab. [Capacitor plates + and −; slab with small dipoles; − charges on the face near the + plate and + charges on the face near the − plate; arrows E and Eₚ]
4. 4 · Opposing field. The surface charges of the slab set up Eₚ opposite to E, so E(effective) = E − Eₚ.
5. 5 · Effect on C. For the same charge, V = Ed falls, so C = q/V rises: C = KC₀ when the slab fills the gap.
6. 6 · Close in one line. So a dielectric lets a capacitor store more charge at the same voltage.

Scoring tip: Name both kinds of molecule with an example each. The first mark usually goes to the definition with an example.