A physical quantity is measured as 12.345 ± 0.005 units. The relative error in this measurement is approximately: MCQ with Answer and Explanation

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.

Explanation:
Wave speed v = frequency (f) × wavelength (λ). Thus v = 50 Hz × 2 m = 100 m/s. This fundamental wave equation applies to all periodic waves (sound, light, water). Memory aid: 'v = fλ – the universal wave equation'. Competitive exams frequently test this direct calculation. Always ensure units: Hz = s⁻¹, so (s⁻¹)×m = m/s, correct for speed. This problem assesses basic wave property understanding, crucial for sound and light topics.

Explanation:
Control rods (made of cadmium, boron, or hafnium) absorb neutrons to regulate the fission chain reaction rate. Inserting rods deeper reduces neutron population, slowing reaction; withdrawing them increases reaction rate. Moderators (water, graphite) slow neutrons to thermal energies for efficient fission. Memory aid: 'Control rods = neutron absorbers; Moderators = neutron slowers'. This application question tests knowledge of reactor components, common in competitive exams. Always distinguish roles: fuel (fissionable material), moderator, control rods, coolant.