When can two objects be in thermal equilibrium?
In a system, if two objects with different temperatures are directly in contact with each other then in time they will have the same temperature reading.
This is because heat transfers from hot to cold objects or from high to low temperature. If no more heat transfer occurs then both objects are in $thermal \ equilibrium$.
Thermal equilibrium is achieved if and only if two systems has the same temperature.
If an insulator such as wood, plastic or glass, is placed in between two objects or systems then it will delay heat transfer or thermal interaction between the systems. An ideal insulator does not permit interaction AT ALL between the two objects. However, if a conductor such as aluminum or copper, then rapid heat transfer occur in the system.
ZEROTH LAW OF THERMODYNAMICS
Consider the following systems inside an ideal insulating wall and using an ideal insulator in between.
- Systems A, B, and C are in thermal equilibrium with each other. Why?
Assuming system A has the highest temperature among the systems then heat can transfer to system C via conductor 1 material then to system B via conductor 2 material. Such that in time, they will achieve thermal equilibrium. This result is called the zeroth law of thermodynamics. - Systems A and B are not in thermal equilibrium with system C. Why?
Assuming system B has the highest temperature among the three systems then heat can transfer via conductor to system A. But heat can't transfer to system C due to the ideal insulating material whether heat comes from system A or B. Thus zeroth law of thermodynamics is not applicable.
Final thoughts
Now, can you explain why the zeroth law of thermodynamics applicable to the following?
