📂Thermal Physics

Definition of Heat in Physics

Definition¹

The energy transferred between two interacting systems is defined as heat.

Description

To understand the definition of heat, it is useful to recall the concept of work. Consider a situation where a person applies a force to move an object at rest. When the object moves, it is said that the force has done work on the object. From the object’s perspective, it has gained kinetic energy, which can be seen as the kinetic energy flowing from the force to the object. In other words, work can be considered as moving kinetic energy. Doing work means changing the object’s kinetic energy, and the flow of heat means a change in the system’s energy.

Therefore, concepts such as the heat contained in an object or high (low) heat do not exist.

Let’s say the energies of two systems $A$ and $B$ are $E_{A}$ and $E_{B}$, respectively. Suppose $E_{A}$ decreases and $E_{B}$ increases. In this case, it is expressed that heat flows from $A$ to $B$. On the contrary, the absence of heat flow means there is no change in energy, and this state is called thermal equilibrium.