what exactly is internal energy.

 

is it constant for every substance????

INTERNAL ENERGY U
Energy Stored in a System at the Molecular Level.
The System's Thermal Energy -the Kinetic Energy of the atoms due to their random motion relative to the Center of Mass plus the binding energy (Potential Energy) that holds the atoms together.

Internal energy is defined as the ENERGY associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale.

For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic.

But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole

Internal Energy is the energy stored in a system at the molecular Level. The System's Thermal Energy -the Kinetic Energy of the atoms due to their random motion relative to the Center of Mass plus the binding energy (Potential Energy) that holds the atoms together in terms of atomic bonds.

We consider all possible internal changes to the body as making up the total internal energy.

There are two ways to change the internal energy: with work, and everything else. Everything else is defined as heat. Heat is the defined as the transfer of energy to a body that does not involve work or those transfers of energy that occur only because of a difference in temperature. As Bellman would say,