The sum of all forms of kinetic and potential energy in a system (Ek+Ep) is described by which term?

• A. Internal Energy
• B. Total Energy
• C. Thermal Energy
• D. Enthalpy

Answer: (D)

Ek plus Ep represents the energy tied to how a system moves and where it sits in a field, which is the mechanical energy of the system. This is separate from the microscopic energy of the molecules themselves, which is internal energy, and from thermal energy, which is related to temperature and also sits within internal energy. Enthalpy, defined as H = U + pV, combines internal energy with the pressure–volume work needed to make room for the system, so it isn’t simply the sum of kinetic and potential energies. The term that best matches Ek + Ep is Total Energy (often used to denote the energy content due to motion and position, i.e., the mechanical energy of the system).