What is thermal equilibrium?

We explain what thermal equilibrium is, what it is and the formula it uses. In addition, the zero law of thermodynamics and examples. what is thermal equilibrium

Definition of thermal equilibrium?

In physics , the state in which two bodies in mechanical contact, or separated by a conductive surface, equals their initially disparate temperatures, due to the irradiation of heat from one to the other until reaching a balance , is called thermal equilibrium. what is thermal equilibrium

If we have two objects in contact, one hotter than the other, as time goes by both will tend to reach the same temperature  and, if there is no loss of heat towards other objects, from now on they will maintain a thermal equilibrium, that is, a temperature constant.

What is thermal equilibrium?

Thermal equilibrium
The thermal equilibrium point occurs when the kinetic energy is distributed in both bodies.

This phenomenon can be explained microscopically, first understanding that the degree of heat (temperature) of the objects has to do with the average kinetic energy of their particles , be they atoms , molecules , or those that should be considered. This average is what is commonly called “internal energy” in physics, so that the greater the kinetic energy, the greater the internal energy and the higher the temperature of the system. what is thermal equilibrium

That kinetic energy being non-stationary (which does not necessarily remain within the object) is possible to understand that two bodies in contact continue to exchange energy as time goes by . And so, the point of thermal equilibrium is reached when the kinetic energy shared between both bodies is distributed throughout the entire system, that is, of both bodies that start to operate as a single thermodynamic system, endowed with the same amount of internal energy and therefore temperature.

Aspects and explanations about thermal equilibrium

Earlier we mentioned that thermodynamics describes equilibrium states from a macroscopic perspective . If we take the state that concerns us from this perspective, we can point out that the particles of the surface in contact of these bodies are capable of interacting with each other. And as we mentioned before, the body with the highest temperature transmits its energy to the one with the lowest temperature. what is thermal equilibrium

This exchange of thermal energy causes the particles of both bodies to have the same energy and temperature. In this way, an exchange takes place that unifies them, reaching the state of thermal equilibrium that this process describes. The temperature of a body is based on the movement of the energy of its particles. what is thermal equilibriumwhat is thermal equilibrium

Therefore, when a body heats up, the explanation is that this heat is the kinetic energy of its particles . The faster the particles acquire, the hotter the temperature of said body will be. This behavior can only be seen with a microscope since we can only notice its temperature if we touch that body.

Thermal equilibrium formula

The expression of the thermal equilibrium is given in degrees Celsius, like any temperature , and is the result of the calculation of the temperature difference between the two bodies, so the amount of heat (Q) that each one loses must first be determined. what is thermal equilibrium

This is determined using the formula Q = m. Ce. Δt , where m will be the mass of the body, Ce its specific heat expressed in lime / gr ° C, and Δt the temperature variation, that is: Δt = tf – ti, final time minus initial time.

Once Q is calculated for each body, we can compare them knowing that the thermal equilibrium occurs in the equality of temperatures between body 1 and body 2, so Q1 = Q2, that is, heat gained = heat lost. The heat that the coldest body gains is that which the hottest body loses .

Heat what is thermal equilibrium

In everyday language, the word heat is related to the sensation in our body when the temperature is high.
In physics , the meaning of heat is different. Heat, in this case, is thermal energy in motion .

The bodies are composed of particles and heat is determined by the degree of agitation of these particles.
That is, the greater the agitation of its molecules, the greater the thermal energy or heat. what is thermal equilibrium

Heat is measured in joules (J) or calories (cal). The calorie is the amount of energy required to increase 1 ° C, 1 g of water. The relationship between these units of measurement is: 1 cal = 4.186 j.


When we need to measure body temperature , we use a thermometer. This device tells us the temperature with thermodynamic equilibrium between the body and the thermometer.

When the thermometer reaches the same temperature as our body, heat transfer ceases and, in some cases, it beeps to warn that the flow has stopped. And so, we know if we have a fever or not.

Forms of Heat Transfer

The thermal energy can be transmitted from one body to another in three ways : conduction, convection and radiation.

Thermal Conduction

Conduction occurs when energy is transferred from molecule to molecule .
The more agitated molecules come into contact with the less agitated molecules and their speed of movement increases.

  • Example: If you hold a metal spoon at one end and touch the other end to the fire, you can burn your hand in no time. This is because the molecules stirred by the fire make the other molecules agitate, carrying heat energy to the entire ladle, increasing its temperature.

Thermal convection

Convection occurs with gaseous or liquid substances, which transfer heat through circular currents that are formed due to the difference in density of the hot and cold parts of a substance.

  • Example: A frying pan that heats water. The part of the water that is closer to the fire will heat up earlier, since the density of hot water is lower, it will rise in the pan and the coldest part will go down, thus forming a circular convection current .

Thermal Irradiation

In irradiation there is no need for contact between bodies for energy to propagate, as occurs in conduction or convection.
In this case, the thermal energy propagates from one body to another using electromagnetic waves . The body that receives heat is called the receiver and the one that emits is called the emitter.

  • Example: This is how the sun heats the planet earth. As we know, the sun is very far from Earth, which means there is no contact. The heat that reaches the earth and guarantees life on the planet is transmitted by radiation through electromagnetic waves.

Zero law of thermodynamics

Thermal equilibrium - zero law of thermodynamics
The zero law of thermodynamics can be expressed as: if A = C and B = C, then A = B.

This principle is expressed in the so-called Zero Law of Thermodynamics, which was expressed in 1931 by RH Fowler as follows: “If two systems A and B are each, separately, in thermal equilibrium with a third system , which we will call C, so A and B are also in thermal equilibrium with each other. ”

That is: if A = C and B = C, then A = B.

Thanks to this principle, centered on the mathematical formulation of thermodynamics, it is known what Maxwell expressed in words in this way: “All heat is of the same type . 

Examples of thermal equilibrium

Here are some simple examples of thermal equilibrium:

  • When we enter a very hot room, we perceive the heat of the immediate air but given a margin of time , our body will get used and will enter into thermal equilibrium with the room, so we will stop perceiving the temperature difference.
  • If we introduce a glass container with cold water into a larger one with boiling water, the temperature flow between the two will cool the hot water and heat the cold, until an intermediate thermal equilibrium level is reached.
  • The products we have in our kitchen’s freezer are in thermal equilibrium with respect to the frozen air between them, so that they all share the same temperature.
  • Thermal equilibrium is reflected in the laws of thermodynamics and is one of the most important principles in physics. The law, which was coined in 1935, says that “if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.”
  • Another example is to throw a cube of ice or very cold water into a container of boiling water . The cold water will warm and the same will happen with the hot water since there will be an exchange of temperature between both bodies. If you have a freezer or freezer, you will know that when you introduce any type of food, it changes its temperature until it is balanced with the environment . This is also known as thermal equilibrium. The same happens when it is defrosted, as it loses cold until it adapts to the temperature of the environment that surrounds it.

Some more best example of thermal equilibrium

The best example to specify thermal or thermodynamic equilibrium is with liquids.
When mixing a hot liquid with a cold one, we have an exchange of energies that involves mixing.

More practical, let’s take coffee with milk as an example. A hot coffee in contact with a cold milk.

Assuming they both have quite different starting temperatures. While the coffee will be 60ºC, the milk, which was in the fridge, is 10ºC.

The mixture, on contact, will tend to heat up. This is because (hot) coffee transmits heat energy to milk (cold body).

As both had specific temperatures (60ºC and 10ºC), the thermal equilibrium will be reached when the mixture reaches 35ºC.

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