When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium. Heat conduction, also called diffusion, is the direct microscopic exchanges of kinetic energy of particles (such as molecules) or quasiparticles (such as lattice waves) through the boundary between two systems. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system. Engineers also consider the transfer of mass of differing chemical species (mass transfer in the form of advection), either cold or hot, to achieve heat transfer. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy ( heat) between physical systems. A hot, less-dense lower boundary layer sends plumes of hot material upwards, and cold material from the top moves downwards. Colors span from red and green to blue with decreasing temperatures. Simulation of thermal convection in the Earth's mantle. Transport of thermal energy in physical systems

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