Heat transfer by direct contact between solids is called:

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

Answer: (A)

Direct contact heat transfer between solids is conduction. When two solids touch, energy moves from the hotter part to the cooler part through particle interactions—molecules colliding and, in metals, free electrons rapidly transferring energy to neighboring particles. In metals, those free electrons act as quick energy couriers, while in insulators the energy travels mainly through lattice vibrations. This process requires a temperature difference and physical contact. Convection needs moving fluids, and radiation transfers energy by waves through space without contact. Absorption refers to taking in radiant energy, not the mechanism of heat transfer by direct contact.