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: For slabs of equal thickness in series, thermal resistances add (R = L/KA), giving an effective conductivity analogous to resistors in series: K<sub>eff</sub> = 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.