Thermal Energy or Internal Energy

In the block-table system simulated by the applet on Page 2, the only kind of external energy transfer to this system is the work Wappl done by the applied force. In the special case in which the block is moving with constant velocity, when the applied force is equal in magnitude and opposite in direction to the friction force, there is no change in the system's mechanical energy. The block's kinetic energy stays constant at a non-zero value and the table's kinetic energy remains constant at 0.

If kinetic energy were the only energy of the block-table system, we would have an example of energy non-conservation: energy is transferred into the system without a corresponding increase in system energy. However, according to the principle of energy conservation, it must be possible to define the total energy of the block-table system so that the change in this energy is equal to Wappl.

This is accomplished by defining an internal energy of the block-table system. Physically, this internal energy is related to how "warm" the system is. The symbol for internal energy is U. The total energy of the block-table system is the sum of the kinetic and internal energy, KE + U.

If there is no change in the system's kinetic energy, one defines the internal energy simply by requiring its change DU to satisfy the energy conservation equation

Wappl = DU,space ifspace DKE = 0.space(1)

More generally, if the kinetic energy KE of the block-table system is changing, one defines DU by

Wappl = DKE + DU.space(2)

Eqs.(1) and (2) assume that no other energy is being transferred to the system, e.g., heat. An example of heat energy transfer is heat conduction from the block or table to the surrounding air. Here we are assuming that such heat conduction is negligible. Otherwise, it must be included in the equations above.

In terms of the molecular model, the internal energy of a body can be explained as the mechanical energy of the molecules constituting the body, both their kinetic energies and the potential energies of interaction between the molecules.