Which statement best describes heat transfer with respect to temperature differences?

• A. It occurs without any temperature difference
• B. It requires a temperature difference
• C. It only occurs in solids
• D. It only occurs in gases

Answer: (B)

Heat transfer needs a temperature difference to occur. Energy flows from hotter regions to cooler ones, and this flow continues until temperatures even out. If the temperatures are the same, there’s no net heat transfer because there’s no driving force pushing energy in any direction. All the common ways heat moves—conduction through solids, convection in fluids, and radiation across space or through a medium—rely on a temperature difference between bodies or regions. Even radiation, which can happen without a surrounding medium, still requires one body to be hotter than the other to transfer energy. So the best description is that heat transfer requires a temperature difference.