• Energy can exist in numerous forms such as thermal, mechanical, kinetic,
potential, electric, magnetic, chemical, and nuclear, and their sum
constitutes the total energy, E of a system.
• Thermodynamics deals only with the change of the total energy.
• Macroscopic forms of energy: Those a system possesses as a whole with
respect to some outside reference frame, such as kinetic and potential
energies.
• Microscopic forms of energy: Those related to the molecular structure of a
system and the degree of the molecular activity.
• Internal energy, U: The sum of all the microscopic forms of energy.