The layer of atmosphere that reflects radio waves is MCQ with Answer and Explanation
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Question #1
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In a nuclear reactor, what is the function of the moderator (like heavy water or graphite)?
Correct Answer: Option A
Explanation:
Uranium-235 undergoes fission most efficiently when struck by slow-moving (thermal) neutrons. However, fission reactions release very fast-moving neutrons. The moderator (like graphite or heavy water) collides with these fast neutrons to slow them down without absorbing them, sustaining the controlled chain reaction. Control rods (cadmium/boron) are used to absorb excess neutrons.
Uranium-235 undergoes fission most efficiently when struck by slow-moving (thermal) neutrons. However, fission reactions release very fast-moving neutrons. The moderator (like graphite or heavy water) collides with these fast neutrons to slow them down without absorbing them, sustaining the controlled chain reaction. Control rods (cadmium/boron) are used to absorb excess neutrons.
Question #2
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An AC generator produces alternating current. To convert an AC generator into a DC generator, which crucial component must be replaced?
Correct Answer: Option A
Explanation:
An AC generator uses continuous 'slip rings' which allow the output polarity to reverse every half-rotation of the coil, generating alternating current. To produce direct current (DC), the slip rings must be replaced by a 'split-ring commutator'. This commutator reverses the connections to the external circuit at the exact moment the coil's current reverses, ensuring current flows in only one direction outside.
An AC generator uses continuous 'slip rings' which allow the output polarity to reverse every half-rotation of the coil, generating alternating current. To produce direct current (DC), the slip rings must be replaced by a 'split-ring commutator'. This commutator reverses the connections to the external circuit at the exact moment the coil's current reverses, ensuring current flows in only one direction outside.
Question #3
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A block of wood floats in water with 60% of its volume submerged. In a liquid of density 0.8 g/cm³, the fraction of volume submerged will be:
Correct Answer: Option A
Explanation:
From floatation: ρ_wood/ρ_water = fraction submerged in water = 0.6. Thus ρ_wood = 0.6 g/cm³ (since ρ_water = 1 g/cm³). In new liquid (ρ_l = 0.8 g/cm³), fraction submerged f = ρ_wood/ρ_l = 0.6/0.8 = 0.75 = 75%. This uses the principle that fraction submerged equals density ratio. Memory aid: 'f = ρ_object / ρ_fluid'. Such comparative buoyancy problems test application of Archimedes' principle across scenarios, common in competitive exams. Always maintain consistent units (g/cm³ here simplifies calculation).
From floatation: ρ_wood/ρ_water = fraction submerged in water = 0.6. Thus ρ_wood = 0.6 g/cm³ (since ρ_water = 1 g/cm³). In new liquid (ρ_l = 0.8 g/cm³), fraction submerged f = ρ_wood/ρ_l = 0.6/0.8 = 0.75 = 75%. This uses the principle that fraction submerged equals density ratio. Memory aid: 'f = ρ_object / ρ_fluid'. Such comparative buoyancy problems test application of Archimedes' principle across scenarios, common in competitive exams. Always maintain consistent units (g/cm³ here simplifies calculation).
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