Radioactivity was first discovered by MCQ with Answer and Explanation

Explanation:
Copper has free electrons, very low resistivity, excellent conductor. Glass, rubber, wood are insulators. Silver best conductor, but copper widely used. Graphite conducts but non-metal.

Explanation:
A potentiometer uses a long wire of uniform cross-section and composition. When a constant steady current flows through it, Ohm's law (V = IR) and the resistance formula (R = rhoL/A) combine to give V = I(rho/A)L. Since I, rho, and A are constant, the potential drop V is strictly directly proportional to the length L of the wire.

Explanation:
Beats (periodic amplitude variation from interference of close frequencies) are used to tune instruments: adjust until beats disappear (frequencies match). Also, in Kundt's tube or other methods, beat frequency helps measure sound speed by comparing known and unknown frequencies. Memory tip: 'Beats: tuning (zero beat = matched frequency); speed measurement (beat frequency = |f₁-f₂|)'. This application question tests wave phenomena uses, frequently appearing in competitive exams. Always link physical phenomena to practical applications; competitive exams emphasize real-world relevance of physics concepts.