In the equation Q = UAΔT_lm, what does U denote?

• A. Overall Heat Transfer Coefficient
• B. Heat Transfer Rate
• C. Temperature Difference
• D. Surface Area

Answer: (A)

U denotes the overall heat transfer coefficient, a measure of how easily heat is transferred per unit area for a given temperature difference across the boundary between two media. It combines all thermal resistances in series—conduction through solids, convection on both sides, and any radiation—so the heat transfer rate is Q = U A ΔT_lm. Its units are W/(m^2·K), and a larger U means heat moves more readily under the same area and temperature driving force. This is distinct from the heat transfer rate itself (Q), the temperature difference (ΔT_lm), or the surface area (A).