Thermal energy

Molecules compose matter, and matter makes up all objects. Thermal energy is the energy contained in the molecules of an object. Thermal energy always flows from an object with more thermal energy to another object with less thermal energy. When more thermal energy is introduced to an object, the molecules in the object vibrate faster. When thermal energy is removed, they vibrate slower. Heat is a part of thermal energy. When people feel heat, they actually feel thermal energy from a warmer object flowing into a colder part of their body. When people feel cold, they feel some of their body's own thermal energy being transferred to another object.

Thermal Energy and States of Matter

The vibrations caused by thermal energy cause matter to change states. Three primary states of matter exist: solids, liquids, and gases. In solids, molecules are closely arranged and tightly pressed against one another. Solids have a definite shape and always take up the same amount of space. In liquids, molecules are bound together, and they flow freely around one another. Liquids have no set shape, but they always take up the same amount of space because the molecules are still bound together. In gases, molecules are free of their bonds. They fly in all directions and bounce off whatever they hit. Gases have no set shape and expand to fill as much space as they can. Ice, water, and steam are good examples of solids, liquids, and gases, respectively.

Whenever one object's molecules come into contact with another object's molecules, thermal energy flows from the object with more energy to the object with less energy. As an example, additional thermal energy causes ice left in a warm room to melt. The molecules of the air have more energy than the molecules of the ice. Thus, the air molecules move faster than the ice molecules. As the air molecules continue to crash into the ice molecules, the air molecules transfer some of their energy to the ice molecules. Eventually, the extra energy causes the ice molecules to break, which changes the ice into water.

How Thermal Energy Moves

Thermal energy flows between objects in three ways: convection, conduction, and radiation. During convection—which only applies to liquids and gases—a warmer fluid (or gas) is exposed to a cooler fluid (or gas). As the two mix, the molecules collide with one another, spreading thermal energy from the warmer to the cooler. Eventually, the mixture has a uniform amount of thermal energy. Air from an open window cooling off a hot room is an example of convection.

Conduction works slightly differently because it relies on contact between molecules. When two solids press against each other, molecules from the hotter one constantly collide with molecules from the cooler one. The collisions force the slower, colder molecules to move faster, which spreads thermal energy from the hotter object to the colder object. Conduction works best with solids and liquids. This is because the molecules in solids and liquids stay closely packed together, so simply pressing the objects against each other is enough to force the molecules to collide. Gases have large gaps between their molecules, which makes causing collisions between molecules extremely difficult.

Metals tend to make the best conductors. They contain special particles that travel through the entire object, moving between the tiny gaps left between molecules. These free-flowing particles collide with all the molecules in the metal, which increases the number of collisions and increases thermal energy. Copper and aluminum—metals commonly used for wiring—conduct thermal energy extremely well.

Convection and conduction both require contact between molecules to spread thermal energy. However, radiation does not; instead, radiation spreads thermal energy through electromagnetic waves. Unlike convection and conduction, radiation can be reflected away from a surface, redirecting some heat that would otherwise be absorbed. In certain circumstances, radiation can even cause thermal energy to pass through an object. Radiation allows the sun to heat Earth, even though most of outer space contains no molecules to transfer its thermal energy.

Stopping thermal energy from flowing keeps warm substances warm and cool substances cool. Insulators stop thermal energy from flowing. Dense objects that allow many collisions to take place, such as metals, transfer too much energy to be effective insulators. Substances with their molecules spread farther apart, such as air and wood, greatly reduce the amount of molecule collisions taking place.

Other insulators work by utilizing a vacuum, or an area with almost no molecules. Because nearly no molecules exist in a vacuum, it is extremely difficult for convection or conduction to occur. Only radiation can transfer thermal energy through a vacuum. A thermos is an example of an insulator. A thermos has an inner wall and an outer wall with a small space in between the two. A special material that reflects radiation covers the walls. The air between the inner and outer walls is removed to create a vacuum. The vacuum stops conduction and convection, and the reflective coating stops radiation. This helps keep liquids stored in a thermos at a constant temperature.

Bibliography

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"What Is Thermal Energy?" IBM, 8 Mar. 2024, www.ibm.com/think/topics/thermal-energy. Accessed 22 Jan. 2025.

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Wysession, Michael, et al. Prentice Hall Physical Science: Concepts in Action with Earth and Space Science. Pearson/Prentice Hall, 2005.