Heat transfer in vacuum occurs by MCQ with Answer and Explanation

Explanation:
For a bipolar junction transistor (BJT) in common-emitter configuration, DC current gain β = I_C / I_B, where I_C is collector current, I_B base current. Typical β values range 20-200. Option D is α (common-base gain). Memory tip: 'β = collector current / base current; large β means small base current controls large collector current'. This definition-based question tests electronics fundamentals, frequently examined in competitive exams. Always distinguish β (CE configuration) from α (CB configuration) and their relationship: β = α/(1-α).

Explanation:
Lactometer is a type of hydrometer used to check purity of milk by measuring specific gravity. Adulteration with water lowers density. It doesn't measure temperature or viscosity directly.

Explanation:
d = (v×t)/2 = (340×0.5)/2 = 170/2 = 85 m. Minimum distance for echo perception is about 17 m. Remember divide by 2 for one-way distance.