The primary principle behind the working of a potentiometer is that the potential drop across any length of a uniform wire carrying a constant steady current is: MCQ with Answer and Explanation

Explanation:
Heat for phase change Q = m·L_f, where L_f is latent heat of fusion. Thus Q = 2 kg × 336 kJ/kg = 672 kJ. This direct application tests calorimetry fundamentals. Memory aid: 'Latent heat: Q = mL; no temperature change during phase transition'. Competitive exams frequently test such calculations with standard values. Always ensure units match (kg and kJ/kg here); convert if necessary. This problem assesses basic formula application skills essential for thermodynamics sections.

Explanation:
Lactometer is a specialized hydrometer for measuring milk purity/density. It works on Archimedes' principle: pure milk has specific density; adulteration (with water) changes density, altering lactometer reading. While it measures density, its specific application is milk testing. Option B is partially correct but not specific; competitive exams expect the precise application. Memory tip: 'Lacto = milk; meter = measure'. This application-based question tests knowledge of scientific instruments in daily life, frequently appearing in competitive exams. Always note context-specific uses of general instruments.

Explanation:
By definition, a force is conservative if the work done by it on an object moving between two points is completely independent of the path taken. A direct consequence of this is that the total work done by a conservative force (like gravity or electrostatic force) in moving an object around a closed loop (returning to the starting point) is exactly zero.