Q4. Assertion: in the ordinary orientational-polarisation model at fixed field and molecular density, the orientational contribution falls as the material is heated. Reason: thermal energy tends to disrupt the alignment of the molecular dipoles with the field. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the extent of polarisation is set by the dipole energy in the field, which aligns the dipoles, against thermal energy, which disrupts the alignment (NCERT page 66).
Q5. Why does a conductor cancel an external field inside its material, while a dielectric slab only reduces it in the fixed-source-charge model?
A conductor’s free charge carriers keep moving until the induced field cancels the external field inside. A dielectric has no (or negligibly few) free carriers; its charges can only shift within molecules or turn with them, and the field of the resulting surface charges only reduces the external field (NCERT page 65).
Q6. A slab of a linear isotropic dielectric with χe = 1.5 has a uniform field of 3.0 × 105 V/m inside it, normal to two of its faces. Find the polarisation and the bound surface charge density on those faces.
P = ε0χeE = 8.854 × 10−12 × 1.5 × 3.0 × 105 ≈ 4.0 × 10−6 C m−2 (NCERT Eq. (2.37), page 66). For a uniformly polarised slab σp = P, so the faces carry ±4.0 × 10−6 C m−2 (see Example 3).