Q4. Assertion: the wires in a house are covered with plastic. Reason: plastics have resistivities 1018 times those of metals or more. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the plastic lets practically no current leak out of the metal (NCERT page 90).
Q5. A wire 2.0 m long with a cross-section of 1.0 mm2 has a resistance of 0.040 Ω. Find its resistivity and say what class of material it is.
ρ = RA/l = 0.040 × 1.0 × 10−6/2.0 = 2.0 × 10−8 Ω m, in the metals’ range: a conductor (NCERT Eq. (3.9), page 84, and page 90).
Q6. Two cubes of the same size are made of materials whose resistivities differ by a factor of 1018. Compare their resistances between opposite faces.
For the same shape, R ∝ ρ, so the resistances also differ by a factor of 1018 (NCERT Eq. (3.9), page 84).