Two parallel wires carrying direct currents in the opposite directions will: MCQ with Answer and Explanation

Explanation:
Carnot efficiency η = 1 - T₂/T₁, where T₁ is source temperature, T₂ sink temperature (in Kelvin). Here T₁=400 K, T₂=300 K, so η = 1 - 300/400 = 1 - 0.75 = 0.25 = 25%. This maximum possible efficiency for given temperatures is a fundamental thermodynamics result. Memory tip: 'η_Carnot = 1 - T_cold/T_hot'. Competitive exams frequently test this formula with varying temperatures. Always use absolute temperatures (Kelvin); Celsius would yield incorrect efficiency. This problem assesses understanding of heat engine limitations.

Explanation:
Bats use ultrasound for echolocation.

Explanation:
Convection: bulk fluid motion carries heat (natural: density-driven; forced: pump/fan). Conduction: molecular collisions; radiation: EM waves. Memory aid: 'Convection = fluid flow + heat; e.g., boiling, weather'. Heat transfer mode frequently tested in competitive exams.