Question

What must be true about two objects if heat is flowing between them?

Answer 1

There is temperature difference between the two objects.

Step-by-step explanation:

If heat is flowing between two objects then there is a temperature difference between the two. One object is at a higher temperature than the other as heat only flows from high temperature to low temperature. Finally when both the objects reaches the same final temperature then heat stops flowing between them. The two objects now has reached a thermal equilibrium.

Answer 2

The objects may exchange heat between each other if they are in contact with each other.

The two objects will allow flow of heat between them if they have a difference in temperature from each other.

The heat will flow from higher temperature object to lower temperature object. The will reach a final temperature (common to each other) and then there will be no heat transfer between each other.