Have you ever wondered why you can feel the heat of a hot drink through the cup, or why your ice cream melts outside of the freezer? These are situations that we tend to take for granted, but their underlying mechanisms are central to the study of thermodynamics.
Simply put, these situations occur because of one fact: thermal energy is transferred from a hotter region to a cooler region, until both regions reach the same temperature, i.e. reach thermal equilibrium. This process is described as the transfer of thermal energy. It occurs through three distinct processes: conduction, convection, and radiation. In this article, we will discuss each of these processes and how they work.
Conduction
Conduction is defined as the transfer of thermal energy during which the medium does not move. This process is usually described in terms of the vibration of particles in solids, for reasons that will become apparent later. When heat is supplied to one end of the solid, the heated particles gain thermal energy, causing them to vibrate vigorously about their fixed positions. These particles then collide with the less energetic neighbouring particles, making them vibrate vigorously as well. This process repeats itself until all particles in the solid are vibrating at the same, elevated rate.
Solids tend to make better conductors of heat compared to liquids and gases. Conduction occurs at a faster rate in solids compared to liquids and gases because their particles are tightly packed. This allows for more frequent collisions, and thus a higher rate of transfer of thermal energy.
Additionally, metals are better heat conductors than non-metals. This is because they contain a high concentration of free electrons. When heated, these fast-moving free electrons quickly diffuse to the cooler regions of the metal, colliding with the particles to transfer their energy. All metals are solid at room temperature with the exception of mercury, which is a liquid. Mercury is a better conductor of heat than most liquids, but a worse conductor than solid metals.
Convection
While conduction can technically occur in all three states of matter, convection only occurs in fluids – liquids and gases. Convection is the transfer of thermal energy due to the movement of the fluid itself. To illustrate this concept, let us consider the example of a beaker of water being heated from its base.
Upon being heated, the fluid at the base of the beaker expands and becomes less dense, relative to the rest of the surrounding fluid. The less dense water then rises to the top, while the cooler and denser water sinks to the bottom. The cooler water, now at the bottom of the beaker, is then heated up, repeating the process. This movement of the fluid as a result of the difference in density is called the convection current.
As is apparent from the example above, convection requires the free movement of particles to occur. That explains why the transfer of thermal energy via convection cannot occur in solids.
Radiation
Radiation is distinct from the other two types of transfer of thermal energy, as it does not require a medium. It is the transmission of energy via infrared waves emitted from the surface of bodies, without the aid of a medium. All bodies emit infrared waves, with hotter objects emitting them at a greater intensity. When bodies absorb infrared waves, their particles gain energy and vibrate more vigorously, causing the temperature to rise.
There are three key factors which affect the rate of radiation of a body:
- Colour and texture of the surface: Dull and darker surfaces absorb and emit infrared radiation at a higher rate, compared to shiny and lighter surfaces.
- Surface temperature: A higher surface temperature in comparison to the surrounding temperature will result in a higher rate of infrared radiation.
- Surface area: A body with a larger surface area will absorb and emit radiation at a higher rate.
Conclusion
The transfer of thermal energy explains a great many things we take for granted today, from cooking to air conditioning. By understanding the concepts of conduction, convection, and radiation, you also gain a better understanding of how the world works.
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