Thermal Equilibrium: Conditions and Application

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A body can be considered in thermal equilibrium if its macroscopic state variables, such as temperature, pressure, and density, remain constant over time. However, if parts of the body are gaining and losing heat simultaneously, it may not be in true equilibrium within the larger isolated system. The heat flow in such cases will vary over time, ultimately leading to a uniform temperature throughout the system. Steady-state conditions can exist, where temperature gradients are stable, but this does not equate to thermal equilibrium. Thus, while a body can maintain certain temperature conditions, it is not in thermal equilibrium if heat exchange is ongoing.
tme92
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Can one say a body which has the conditions above mentioned be in thermal equilibrium? How does one apply the definition of temperature equilibrium to this situation?

(Physically I think it's possible for a body to be in this situation if some part of it receives heat from the outside and another loses heat)
 
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Interesting question. I guess it's a matter of what you mean by thermal equilibrium. In many textbooks they say thermal equilibrium is when the macroscopic state variables (temperature, pressure, density,...) of the system are constant over time. In this sense you can say that your system is at equilibrium. But if you consider the whole isolated system your subsystem belongs to, this is not at equilibrium. The flow of heat between all the parts of the isolated system cannot be steady forever, but will forcefully vary over time, and finally vanish when all the parts of the complete system will be at the same temperature (this because an infinite heat reservoir doesn't exist).
 


tme92 said:
Can one say a body which has the conditions above mentioned be in thermal equilibrium? How does one apply the definition of temperature equilibrium to this situation?

(Physically I think it's possible for a body to be in this situation if some part of it receives heat from the outside and another loses heat)

The body is not in equilibrium, but may have steady-state conditions. Our bodies have these conditions: core temperature 98.6 F, skin temperature closer to ambient air temperature.

All this means is that there is a steady flow of heat, and T(x,t) = T(x).
 
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