The scientific principle which explains the observation that the amount of heat transfer accompanying a change in one direction is numerically equal but opposite in sign to the amount of heat transfer in the opposite direction is:

The Law of the Conservation of Energy

Explanation:

The Energy Conservation Act tells us that any form of energy changes from one form to another. And that you can never have a higher amount of energy than you originally got. That is, if you have 100 [J] of energy to be transferred as heat from one medium to another, only 100 [J] can be used to be transferred, no amount greater than this value can be transferred.

The particular study of Energy Transfer in the form of heat is important in sciences such as thermodynamics and heat transfer.