What is a dielectric? Distinguish between polar and non-polar molecules. Explain, with a labelled diagram, how a dielectric slab reduces the electric field inside it.

1. 1 · Define a dielectric. An insulator with no free charges; a field can only shift its bound charges slightly.
2. 2 · Polar vs non-polar. Polar molecules (H₂O) have a permanent dipole; non-polar ones (O₂) get an induced dipole in a field.
3. 3 · Explain polarisation. In a field the dipoles line up; inside they cancel, leaving −σₚ and +σₚ on the two faces.
4. 4 · Draw the slab. [Field E₀ from left to right; slab with − signs on the left face and + signs on the right face; smaller arrows inside labelled E = E₀/K]
5. 5 · State the result. The surface charges oppose E₀, so the field inside falls to E₀/K, where K = ε/ε₀.
6. 6 · Real life. Water (K about 80) weakens the attraction between ions, so salt dissolves in it.
7. 7 · Close in one line. A dielectric weakens, but never cancels, the field inside it.

Scoring tip: In the diagram, put the negative charges on the face where the field enters. Swapping them is a common mistake.