# Do bodies need to be in contact to be in thermal equilibrium?

In this wikipedia article, it is written that:

Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in thermal equilibrium with itself if the temperature within the system is spatially uniform and temporally constant

This is alright, but how exactly are we defining a 'path' here? Does the two body have to be in physical contact, because you could transfer heat through methods other than physical contact.

On some further digging, I found this Physics libre text chapter discussing this same point (here), it is said that

An important concept related to temperature is thermal equilibrium. Two objects are in thermal equilibrium if they are in close contact that allows either to gain energy from the other, but nevertheless, no net energy is transferred between them. Even when not in contact, they are in thermal equilibrium if, when they are placed in contact, no net energy is transferred between them. If two objects remain in contact for a long time, they typically come to equilibrium. In other words, two objects in thermal equilibrium do not exchange energy.

So, does this imply that for thermal equilibrium needs only transfer by 'conduction' as zero? If so, why do we neglect convection and radiation methods of heat transfer?

• from the second reference you quote, "Even when not in contact, they are in thermal equilibrium if, when they are placed in contact, no net energy is transferred between them", seems to be stating rather clearly that being in contact is not a necessary condition at all Jan 4, 2021 at 10:35