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What is Thermal Equilibrium?

Thermal equilibrium occurs when two objects in contact no longer exchange heat, reaching a uniform temperature. It's a dance of energy, where molecules settle into a harmonious state, neither gaining nor losing warmth. This balance is fundamental in understanding not just physics, but the everyday world around us. How does this concept impact your daily life? Let's explore together.
Daniel Liden
Daniel Liden

Thermal equilibrium is achieved when two objects or systems reach the same temperature and cease to exchange energy through heat. When two objects are placed together, the object with more heat energy will lose that energy to the object with less heat energy. Eventually, their temperatures will be equal and they will cease to exchange heat energy as neither object is warmer or cooler than the other. At this point, they are in a state of thermal equilibrium.

Thermal contact is an important concept relating to thermal equilibrium. Multiple systems are considered to be in thermal contact if they are capable of affecting the others' temperatures. If a bottle of soda is removed from the refrigerator and placed on the kitchen counter, which is at room temperature, the counter and the bottle of soda are in thermal contact. Heat energy from the counter flows to the cold bottle of soda. Eventually, their temperatures will be equal and they will be in a state of thermal equilibrium.

Scientist with beakers
Scientist with beakers

In thermal systems involving objects in thermal contact, heat flows from the warmer object, which contains more thermal energy, to the cooler object, which contains less thermal energy. Therefore, objects and systems can either gain or lose heat. They technically can not gain or lose cold, though, as there is no separate “cold energy.” In a thermal system, the warmer object loses heat energy to the cooler object until equilibrium is achieved.

The concept of thermal equilibrium is most often used and studied in the field of chemistry, particularly in the study of thermodynamics. Thermodynamics is usually taught at an introductory level in high school chemistry courses, but it is a very complex topic that is not fully explained to students until advanced chemistry and physics classes in college.

Along with its place in the lab, thermal equilibrium has a very important place in nature. Chemical systems exist in many living and nonliving things in nature, and keeping those systems in a state of equilibrium is very important. In the human body, for example, enzymes are involved in countless chemical reactions that are vital to life. Enzymes, however, become useless when they reach too high or too low of a temperature. They must remain at a constant equilibrium to remain functional, and they must remain functional for the human body itself to function.

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Discussion Comments


How can you find out how long it takes for two objects to reach thermal equilibrium?


Yes, please explain, when thermal equilibrium has been reached, are there still heat transfers between the two bodies to maintain that equilibrium or does transfer completely stop? --Saborknight


when thermal equilibrium is achieved, there's no heat transfer anymore between the two bodies. is it true? can you explain a little bit more detail?


@ Babalaas - Great questions! To answer your first question "does thermal equilibrium...explain condensation", the answer is yes and no. Condensation forms due to the thermal properties of gases. Air has some moisture in it, and it wants to contract as it gets colder and expand as it warms. Thermal equilibrium causes air to heat and cool but the contraction of the air, and the subsequent gathering of the water molecules, is what causes the condensation to form on the cold surface of the soda bottle. Now I will try to answer your second inquiry. I love to cook, so the first example of thermal equilibrium that comes to mind would be thawing foods and heating water. Thermal equilibrium explains why it is quicker to thaw food in ice water as well as boil water hot from the tap rather than cold. The greater the difference in the temperature of two items the more energy that must be exchanged to create equilibrium. Basically the more energy exchanged the longer to reach equilibrium if time is a constant.


Wow, I never realized how in-depth the human understanding of heat is. I am not that scientifically inclined, so this article was very interesting to me. Does the thermal equilibrium property explain how that same bottle of soda placed on the counter top begins to condensate as it warms to room temperature? What are some other real world examples of how thermal equilibrium works? This stuff is so interesting it makes me want to brush up on my chemistry. It has been so long since I have studied this stuff.

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      Scientist with beakers