When air in a room receives thermal energy from a heater, the air molecules gain kinetic energy and begin to move faster, causing them to spread apart and decrease in density. This lower density air becomes less dense than the surrounding cooler air in the room and therefore rises, creating a convection current. As the warm, less dense air rises, cooler, denser air from the surrounding area moves in to take its place, creating a continuous circulation pattern of warm air rising and cool air sinking. This process is known as convection and is the reason why hot air rises in a room when it receives thermal energy from a heater.
