If there is no forced movement in the surrounding air (or water, etc), then before too long the air gets warm. Quasi equilibrium is reached when there is a stable thermal gradiant in the air surrounding the heat sink. Heat is then primarily transfered via diffusion/convection through the air, as well as the afore mentioned radiation.
Not quite, although I doubt you left the following out because you didn't know about it :-). If there is no forced convection (i.e., a fan blowing air across the fins of a cooling element) then as the air heats up, you get
natural convection because warm air is lighter than cold air and thus begins to move upwards. This process only works in the presence of a gravitational field; in other words, natural convection doesn't arise in space. You get very weird effects, as illustrated by this picture of a burning candle in a low-g environment:
Natural convection is a very inefficient way of generating an air flow, and thus is of little practical importance in cooling of earth-based applicances. At room temperature, a rule of thumb says that the magnitude of heat dissipation by natural convection is approximately equal to that of radiation, but as the latter is proportional to the temperature of the hot object to the
fourth power (!), radiation quickly becomes the more important mechanism.
Neither can hold a (figurative) candle to forced convection, however, unless the temperature is
really high, but by that time you no longer have working computer.
