Definitions

# Thermal Equilibrium

Thermal equilibrium is a thermodynamic condition in which two or more bodies are at the same temperature . This condition is achieved spontaneously , since bodies at different temperatures exchange heat with each other until their temperatures equilibrium. Also, heat always flows from hotter bodies to cooler bodies.

## Principle

The physical principle that explains this phenomenon is called the Zeroth Law of Thermodynamics . Check the wording of this law, in a simplified form:

“Consider two objects A and B. If a third object T is in thermal equilibrium with A and also in thermal equilibrium with B, then A and B are in equilibrium with each other.”

The following figure illustrates the physical principle of it. If bodies A, of temperature T A , and B, of temperature T B , are in thermal equilibrium with body C, of ​​temperature T C , then: A = T B = T C .

## Heat and thermal equilibrium

It occurs when the components of a thermodynamic system transfer heat to each other, always in the direction of the hotter body to the colder body , until their temperature is balanced.

For thermal equilibrium to occur, the bodies must be in thermal contact Thermal contact is characterized by the ability of bodies to exchange heat with each other. This heat, in turn, can either be latent or sensible . If you want to know more about these forms of heat, visit: Sensible heat and Latent heat .

### Formula

As a result of the conservation of energy, we can say that, in a closed thermodynamic system (which does not exchange heat with its surroundings), all the heat that is transferred by a body is fully absorbed by the others. In this way, we can equate the following identity:

C — heat given up

The previous identity can be generalized , with this we can state that the sum of all heat quantities transferred between bodies, in thermodynamically closed environments , is zero :

## Earth’s thermal equilibrium

It is fundamental to our existence and that of other species. Without the occurrence of this phenomenon, the Earth would not have the necessary conditions for the maintenance of life . Due to their high light reflection capacity , the seas and clouds reflect part of the solar radiation , while the rest of this radiation penetrates the earth’s atmosphere , warming the planet .

Due to the presence of greenhouse gases ( carbon dioxide , methane , nitrous oxide), solar radiation is continuously reflected to the Earth’s surface, propagating thermal radiation across the globe. Much of this thermal radiation is also consumed as latent heat, melting the ice present in the clouds. Without this energy equilibrium, the thermal amplitude on our planet would be enormous: we would have scorching days and extremely cold nights.

Despite being responsible for the planet’s thermal equilibrium, greenhouse gases are thermal insulators and act as heat retainers. This means that much of the heat that should have escaped out of the Earth’s atmosphere remains on Earth, causing increasing increases in its average temperature .