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Singapore’s hot and humid climate makes thermal energy something we experience every day. From the heat radiating off sunlit pavements to the cool air produced by an air-conditioner, thermal processes are constantly at work around us. But how does energy cause objects to become hotter or cooler, and how is this energy transferred from one place to another? In this article, we will explore the internal energy store, how this is linked to temperature, and the different ways thermal energy is transferred in everyday life.
Thermal energy refers to the energy stored within a substance due to the motion and arrangement of its particles. To be precise, it is the total internal kinetic energy and internal potential energy of the atoms or molecules that make up the substance. Thermal energy is measured with the SI unit Joules (J). Thermal energy can be transferred from one object to another, from a hotter object to a colder one.
According to the kinetic theory of matter, particles are always in constant random motion. The faster these particles move on average, the higher the substance’s temperature. The forces and distances between particles also contribute to their internal potential energy. Temperature is measured using the SI unit Kelvin (K), but also commonly measured in degree Celcius (°C) and Fahrenheit (°F).
Fun fact: To convert °C to K, simply add 273.15 to the temperature in °C.
Since temperature is a measure of the average kinetic energy within an object, it is directly related to how much energy is within the internal store of an object.
When an object is being heated, energy is being transferred from an object of higher energy (higher temperature), to another object with lower energy (lower temperature). This transfer of energy continues until both objects are at the same temperature. At this point, both objects are in thermal equilibrium, where there is no net transfer of energy between them. This energy transfer can occur in three ways: Conduction, Convection and Radiation.
Conduction is the transfer of thermal energy through a substance, or between objects in direct contact, without any bulk movement of the material. When one part of an object is heated, its particles gain kinetic energy and move or vibrate more vigorously. These “hotter” particles then collide with neighbouring particles, transferring some of this energy to them.
This chain of collisions passes thermal energy from the hotter region to cooler regions of the object. The transfer continues until the object reaches a uniform temperature, at which all particles within the object are in thermal equilibrium.
Conduction transfers thermal energy most effectively in solids because their particles are closely packed. This allows energy to be passed quickly from one particle to a neighbouring particle through collisions and vibrations. In liquids and gases, particles are farther apart, so collisions happen less frequently, and conduction is much less effective.
Different solids also conduct thermal energy at different rates, depending on their material. Metals are excellent thermal conductors because they contain free electrons, which also facilitate thermal energy transfer through conduction. Non-metals such as wood, plastic, and rubber do not have these free-moving electrons, so they are poorer conductors and are often used as thermal insulators.
This explains why a metal surface feels cooler than a wooden surface even when both are at the same temperature. Metal conducts thermal energy away from your hand much faster than wood, causing your skin to cool more quickly.
Convection is the transfer of thermal energy through the bulk movement of a fluid, such as a liquid or gas. It cannot occur in solids because their particles are fixed in position.
When a fluid near a heat source is heated, its particles gain kinetic energy and move farther apart. The fluid expands, becoming less dense than the cooler fluid around it, so it rises. Cooler, denser fluid then sinks to take its place near the heat source.
This cooler fluid is heated and rises in turn, while the previously heated fluid cools as it moves away from the heat source, becomes denser, and sinks again. This continuous cycle of rising warm fluid and sinking cool fluid is called a convection current, transferring thermal energy throughout the liquid or gas.
Radiation refers to the transfer of thermal energy through electromagnetic waves, particularly infrared radiation. Unlike conduction and convection, radiation does not require a medium and can take place in a vacuum. That’s how thermal energy from the sun crosses space to reach us on Earth!
Objects can emit, absorb, or reflect thermal radiation. Emission is the release of thermal energy as electromagnetic waves, while absorption occurs when a surface takes in this radiation and gains energy. If radiation is reflected instead, it bounces off the surface without transferring energy to it.
A hotter object emits more radiation than it absorbs, resulting in a net emission of radiation and drop in temperature. Likewise, a cooler object absorbs more radiation than it emits, resulting in a net absorption of radiation and increase in temperature.
The rate of thermal radiation depends on the colour and texture of a surface, its temperature, and its surface area. Dark, rough, and dull surfaces are good absorbers and emitters of radiation but poor reflectors. In contrast, light‑coloured, smooth, and shiny surfaces reflect radiation well but are poor absorbers and emitters. A hotter and larger surface emits more radiation than a cooler and smaller one
Temperature is regulated through the transfer of thermal energy between objects, so you can see examples of conduction, convection, and radiation all around you.
When cooking, you want the pan to heat up quickly, so the body is made from a highly conductive material such as metal. However, you also need to avoid burning your hands, so the handle is made from a thermal insulator like wood.
In sunny Singapore, we rely heavily on air‑conditioning to cool our rooms quickly. But why are conventional air‑con units always mounted near the ceiling? We’ve just learned how a convection current forms when a fluid is heated. The same idea applies to cooling. Air that has been cooled by the air‑con becomes denser and sinks to the bottom of the room, where we are, while warmer, less dense air rises to the top to be cooled again. This continuous cycle of cool air sinking and warm air rising creates a convection current that circulates cooled air throughout the room.
A familiar example of how surface properties affect radiation is household aluminium foil, which has a shiny side and a dull side. The shiny side is slightly more reflective and slightly poorer at emitting and absorbing than the dull side, so in principle it can reflect a bit more of the thermal radiation emitted by warm food back towards the food, helping to keep it warm. Conversely, the dull side is better at absorbing and emitting radiation, so it can take in a bit more radiant energy from an oven and transfer it to the food.
In practice, however, the difference between the two sides is very small. So while the physics is real, it usually doesn’t make a noticeable difference in everyday cooking, though it’s still a neat example of radiation in action.
In this article, we’ve seen how thermal processes transfer energy between internal stores in objects, and how these processes produce the very real temperature effects we experience every day. The next time you feel overwhelmed by the tropical weather, remember that it is these elementary processes of energy transfer (conduction, convection, and radiation) that shape the world around us.
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Prepared by: Michelle
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