Internal energy E

• Energy contained within the system,
• • excluding the macroscopic kinetic energy of motion of the system as a whole
  • excluding the potential energy of the system as a whole by the external force
• Absolute value is arbitrary
• State valuable
• • uniquely determined by the state
  • independent from the path and hysteresis

The 1st law of thermodynamics

• The internal energy changed by
• • Energy transfer by heat 𝑄
  • • Convention: from the outside to the system is (+)
  • Energy transfer by work −𝑊
  • • Work from the outside to the system -> the system is compressed -> minus sign of work (-)
• ΔE = Q −W
• • In a differential formula, d𝐸 = d𝑄 −d𝑊 = d𝑄 −𝑃d𝑉
• This law is actually the law of energy conservation.
• • Energy added to matter by heat and work does not disappear, but is stored as internal energy.
  • If the matter give energy by heat or work, such energy is taken from the matter,  its internal energy decreases.