Answer: 2K₁K₂/(K₁ + K₂).
- A (K₁ + K₂)/2
- B 2K₁K₂/(K₁ + K₂)
- C K₁K₂/(K₁ + K₂)
- D K₁ + K₂
Correct answer: B. 2K₁K₂/(K₁ + K₂)
Explanation: Rods in series add thermal resistances, giving the harmonic mean form: 2K₁K₂/(K₁ + K₂).
During a phase change (melting or boiling), temperature stays constant while heat is absorbed entirely as latent heat (Q=mL); temperature only rises again once the substance is fully in its new phase.
Concept context
Temperature scales, thermal expansion, calorimetry, and the three modes of heat transfer including radiation laws.