Question

explain how all the air in room gets heated when a room heater kept on the floor in a corner of the room is switched on

Answer 1

as if we close the room and open the gate of fridge so the room will quickly get cooled , so in the same way when heater  is on the heat rises in the cool air because it is less dense . as the there are three ways of passing the heat :
convection , conduction , radiation . so a convection heater blow warm air,
and air is lighter so the room get heated immediately 

Answer 2

all the air in room gets heated when a room heater kept on the floor in a corner of the room is switched on because the hot air moves up and cold air moves down after somtime all the cold air changes into hot air