The First Law of Thermodynamics - Energy is conserved.

The internal energy of a system is the sum of all the kinetic and potential energies of all its components.

ΔE = E_final − E_initial

The equation above indicates the change in internal energy, ΔE, is the difference between the final energy, E_final, and the initial energy, E_initial.

Thermodynamic quantities like ΔE have three parts: a number, a unit, and a sign.

• The number and unit give the magnitude of the charge.
   
• The sign gives the direction.
   
  • A positive ΔE, when E_final > E_initial, indicates the system gained energy from its surroundsings. (diagram B below)
     
  • A negative ΔE, when E_final < E_initial, indicates the system lost energy to its surroundsings. (diagram A below)