A bimetallic strip is used in MCQ with Answer and Explanation

Explanation:
Average Earth-Sun distance (1 AU) ≈ 1.5×10¹¹ m. Speed of light c = 3×10⁸ m/s. Time t = distance/speed = 1.5×10¹¹ / 3×10⁸ = 500 seconds ≈ 8.33 minutes ≈ 8 minutes. This is a standard astronomical fact. Memory aid: 'Sunlight takes 8 minutes to reach Earth; we see the Sun as it was 8 minutes ago'. This factual knowledge question tests awareness of scale in physics, frequently appearing in competitive exams. Always recall key constants: Earth-Sun distance, light speed, for quick estimations.

Explanation:
Conservation of momentum applies to isolated systems with no external forces. In billiard ball collision on frictionless table, external forces (gravity, normal) cancel vertically, and no horizontal external forces act, so horizontal momentum is conserved. Book on table has zero momentum but no dynamic process. Falling ball has external gravity force. Accelerating car has external engine force. Thus only option B represents an isolated system where momentum conservation holds. Memory aid: Momentum conserved when net external force = 0. This conceptual question tests understanding of system boundaries and force analysis, crucial for mechanics sections in competitive exams.

Explanation:
The thermal efficiency (η) of a reversible Carnot engine depends exclusively on the absolute temperatures of the hot source (T1) and the cold sink (T2). The formula is η = 1 - (T2/T1), or (T1 - T2) / T1. Temperatures must be in Kelvin. It shows no engine can be 100% efficient unless the sink is at absolute zero.