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.

  1. What is 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.

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.

  1. 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 .

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.

  1. 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.

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 .

  1. 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 . 

  1. 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.

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