By radiation, heat propagates without leading to a movement of matter, only the mechanical energy waves or electromagnetic energy waves move. Heat radiation propagates through gases, often by air. Through convection, takes place a movement of substance, fluids, liquids or gas from bottom up and vice-versa. Hot particles become lighter and rise, their place being taken immediately by the cold particles. The hot particles now are far from the heating source, they release the heat and become cold and heavy again, going down to the heating source level. In this way the cycle continues.
What do we understand here by “particle”? A gas molecule? Certainly not. The “particle” means a bunch of gas molecules joined together by forces of cohesion. As the particle is hotter, the more cohesion forces are weaker (Force Vector reduced in module). The “particle” becomes lighter (There are fewer gas molecules in the same space).
When we talk about heat propagation by contact, everything is the same as heat propagation by radiation, just wave energy is moving, without triggering a movement of the substance. The main difference is that the substance is solid and the wave propagates in all directions through the oscillatory motion of the particles of the solid substance, especially movements of ions.
A very conclusive experiment is the gravitational pendulum row. The experiment helps us to understand the phenomenon of wave energy propagation through the oscillatory motion.
Oscillatory motion can be considered as a projection of material point motion from circular motion. One complete oscillation equivalents to one complete rotation of a material point. The material point, as an abstract notion introduced in Physics is the mobile from the classical Mechanics or Newtonian which has no dimensions and no mass, so it doesn’t influence the movement elements.
The movement, the mechanical work and the heat are strongly related to each other. The measurement unit Joule is used for both mechanical work and heat. For heat, sometimes is used a derivative measurement unit called calorie. Heat radiation that is coming from a radiator for example, takes place through the oscillation of the ions which make the metallic compound of the material they composed.
The radiation by air starts only when the energy waves reach the end of the metallic material, until then was just a propagation of heat by conduction. When the heat propagates through same material we call it “propagation of heat by conduction”. When the heat propagates through two or more materials placed in contact then it is called “propagation of heat by contact”. In fact, this is the same mode of propagation of heat, there are not two different method.