In a vacuum, which heat transfer mode is used between surfaces?

• A. Conduction
• B. Evaporation
• C. Convection
• D. Radiation

Answer: (D)

In a vacuum, heat transfer between surfaces occurs by radiation because there’s no medium to carry heat by contact or flow. Conduction needs a solid, liquid, or gas to transfer kinetic energy through particle interactions, and convection requires a fluid that can circulate and move heat; neither is present in a vacuum. Radiation, on the other hand, uses electromagnetic waves to move energy directly from the hotter surface toward the cooler one, no matter the empty space between them. The amount of heat radiated depends on temperature, surface area, and how well the surfaces emit and absorb radiation (emissivity), with the relationship described by the Stefan-Boltzmann law (P = εσAT^4 for a blackbody, with real surfaces incorporating emissivity and view factors).