The situation where a room is heated as a result of solar
radiation coming in through the windows is a heat interaction since it is due
to the temperature difference between the sun and the room that caused the
heat.
By a virtue of a temperature difference due to thermal
interactions, energy in the form of heat is transferred between a subsystem and
its surroundings. Mass flow are the main participants of thermal interactions.
However, this conversation about thermal interactions is confined to close
subsystems only, i.e. zero mass flow subsystems opposite their boundary.
<span>
Radiation and conduction are the two difference ways where heat can be
transferred thru a subsystem boundary.</span>
