As we learned in the previous lesson, if there is a temperature difference between systems and surrounding, energy transfer occurs and we call it heat. And we also acknowledge that the colloquial term for energy transfer is the redundant term ‘heat transfer’. So in a nutshell, heat transfer is all about the transfer of energy and the means how they are transferred.
- Conduction - Transfer of energy via molecular contact in rigid bodies.
- Convection - Energy transfer via the movement of fluid.
- Radiation - Energy transfer due to electromagnetic radiation.
For example, consider a kettle on a stove. The flame transfers energy to the pot and the kinetic energy of the molecules gets transferred to other molecules in contact, thereby heating the fluid inside. We call this thermal conduction. And when the liquid boils, there is a density gradient from the top to bottom of the liquid and this creates a flow of molecules. Also the hot air vapour expands into the ambient air, transferring energy as well. This transfer of energy due to mass movement of fluid is called as thermal convection. Also, at the same time the flame itself is radiating energy in the electromagnetic spectrum and transferring some of its energy. This is known as thermal radiation.
In the above example, the molecules of air from the flame has high kinetic energy. So at the molecular scale, the hot molecules hit the less hot molecules of the pot and transfer their kinetic energy. This contact type energy transfer is called as conduction.
When the pot gets hot and via conduction heats the fluid, say water, it boils at a certain temperature. And in that fluid, you’ll have the hotter molecules expanding and lowering the density at the bottom. But at the same time the warmer and denser molecules at the top races to the bottom due to the difference in the density. And this transfer of energy from one point to the other in a medium is known as convection. This mass movement of fluid is called convection.
Matters that are above the absolute zero, radiate energy in the form of electromagnetic waves. Even the universe in its entirety radiates energy in the microwave spectrum known as the cosmic microwave background. A well known example for thermal radiation in visible spectrum is any iron object heated enough that it glows ‘red-hot’.
Knowledge of heat transfer is what helps us make things like heat exchangers, cooling systems, and many more interesting things that we’ll be exploring in an upcoming Mindspace course on practical thermodynamics.