A warm hand is placed in water at 70°F (21°C). Heat is transferred from the hand to the water by which process?

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

Answer: (B)

When two substances are in direct contact, heat flows from the hotter to the cooler one mainly through conduction—the transfer of energy by molecule-to-molecule interaction at the interface. In this situation, a warm hand sits in water that’s cooler, so heat moves from the hand into the water through the contact surface. Water is a fairly good conductor of heat, so this conduction is efficient. As the water near the hand warms, convection within the water can help spread the heat, but the initial transfer mechanism at the boundary is conduction. Evaporation and radiation don’t describe the primary transfer here, and while convection can occur in the water, it’s not the initiating process at the hand-water interface.