The working principle of LED is based on: MCQ with Answer and Explanation

Explanation:
Rocket propulsion: expelled gases backwards impart forward momentum to rocket. Total momentum of rocket + exhaust gases remains conserved. No external force, so momentum conservation applies. Jet engines also work on similar principle.

Explanation:
Most solar cells use silicon (monocrystalline or polycrystalline). Other materials: gallium arsenide, cadmium telluride. Silicon is semiconductor, abundant, efficient. Converts sunlight to electricity.

Explanation:
Work done = F·s = Fs cosθ. Coolie applies vertical force to support weight, displacement horizontal, angle 90°, cos90°=0, work zero. No displacement of force direction. Pushing wall: displacement zero, also zero work. But option A is standard example. Pushing wall also zero, but question expects one answer. I'll adjust: I'll remove 'pushing a wall' to avoid ambiguity. I'll replace with 'A satellite revolving around Earth in circular orbit' etc. However, here I'll keep A, note that pushing wall also has zero displacement, but the question lists specific cases, only one may be correct. Actually, in pushing wall, wall doesn't move, work zero, but often they consider pushing wall as no work. However, option D 'Pushing a wall' might also be zero. To avoid two correct, I'll change D to 'Throwing a ball upward' so only A is zero. I'll modify the question: D: 'Throwing a ball upward' (work positive). So answer A. Let's do that.