Chem 1000: The First Law of Thermodynamics

When considering the energy of a system, we use the notion of internal energy. INTERNAL ENERGY, U is the total energy (both kinetic and potential energy) in a system. A system only contains internal energy. A system does not contain energy in the form of heat or work. Heat and work only exist during a change in the system. The relationship between these three factors can be established through the FIRST LAW OF THERMODYNAMICS which is:

Note that an isolated system is unable to exchange work or heat is thus Change in internal energy in a system is equal to 0. The energy of an isolated system is constant

To use the above equation, certain points must be considered:

• Any energy entering the system carries a positive sign. Thus if heat is absorbed by the system, q > 0 and work is done on the system, w > 0
• Any energy leaving the system carries a negative sign. Thus if heat is given off by the system , q < 0 and work is done by the system, w < 0
• In general, the internal energy of a system changes as a result of energy entering or leaving the system as heat or work. If more energy enters than leaves, the change in internal energy is positive. If more energy leaves than enters, change in internal energy, the change in internal energy is negative
• In isolated systems, energy is conserved and displayed through the equation:
  U_system = - U_surroundings