How does heat get from the stove burner into your soup?
Transfer of thermal energy by convection.
There are three main methods for heat to transfer:
- Conduction - occurs when warmer objects are in physical contact with colder objects.
- Convection - when colder, denser fluid falls and pushes up warmer, less dense fluid.
- Radiation - movement of thermal energy as electromagnetic waves.
In this situation, convection is happening. Convection applies the properties of density, where less-dense objects rise to the surface when submerged in a fluid-like substance. We also know that hot air expands, thus becomes less dense than the air around it.
Applying those two "laws" into convection, when the soup near the bottom of the stove heats up, it moves in a convection current where the warmer fluid rises/pushes up (because it's less dense). That said, the warmer soup displaces the colder soup and the process repeats. Eventually, your soup is hot throughout.
Hope this helps :)