Transmission of Heat
Heat is transmitted primarily in three different ways:
  • Conduction, 
  • Convection,
  • Radiation.  
Conduction is a slow process where the molecules of an object transfer heat by "touching" those of a colder object. 
This method requires the presence of a medium and it is the typical transfer method of heat between solids and liquids or both. 
Convection occurs when the "hot" molecules of an object transfer the energy to the air molecules surrounding it and in turn these will carry the heat to the cooler spaces beyond. 
Convection requires the presence of a medium as well and cannot occur in empty space or vacuums.  
Convection occurs usually between (fluids) liquids and gases or liquids and other liquids 
Radiation can take place where a hot object send heat in the form electromagnetic waves through empty space or through a vacuum. 
This is the case of the heating effect between the Sun and the Earth.
Consider the picture below:
A small pot with milk in it is placed on a stove:
The following inferences can be made:
  • Conduction: occurs between the hot stove element and the cooler bottom of the pot
  • Convection: occurs between the hot milk (water vapour) and the cooler air molecules surrounding the top of the milk surface