A voltmeter is always connected in MCQ with Answer and Explanation

Explanation:
LED (Light Emitting Diode) works on electroluminescence: when electrons recombine with holes in a semiconductor junction, energy is released as photons. This is distinct from incandescence (heating filament, like bulbs), fluorescence (absorb UV, emit visible), or heating effect. Memory aid: 'LED = semiconductor light emission; no filament'. This modern application question tests knowledge of everyday physics, common in competitive exams. Always link device to underlying phenomenon: LED (electroluminescence), laser (stimulated emission), solar cell (photovoltaic effect).

Explanation:
Elastic PE: U = ½kx² ⇒ k = 2U/x². x = 3 cm = 0.03 m. k = 2×18/(0.03)² = 36/0.0009 = 40000 N/m. Unit conversion critical: cm to m. Memory aid: 'k = 2U/x²; always use SI units'. Tests spring energy formula application with unit conversion, frequently appearing in competitive exams.

Explanation:
Resonance in series LCR circuit occurs when X_L = X_C, i.e., ωL = 1/(ωC) ⇒ ω = 1/√(LC). At this frequency, impedance is minimum (Z=R), current maximum, and circuit is purely resistive. Memory tip: 'Resonance: X_L = X_C; ω₀ = 1/√(LC); series: Z_min, parallel: Z_max'. This AC circuit concept is frequently tested in competitive exams. Always distinguish series and parallel resonance conditions; competitive exams often test series LCR resonance fundamentals.