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Diversions

Professor Hibbert's Perpetual Motion Pages

The First Law of Thermodynamics

The energy of an isolated system is constant

or

dU = dq + dw

(The change in internal energy = heat exchanged with surroundings + work done on system)

A 'system' is whatever you are studying when you apply thermodynamics to a problem. If you apply thermodynamics to an object in motion then the object is the system. If you apply thermodynamics to a steam engine, the steam is the system, and if you apply thermodynamics to chemical reactions, the system is the matter (atoms and/or molecules) undergoing the chemical reaction.

The system is made up of atoms and these atoms will be moving, at least because of the thermal energy of the system, and possibly because the system as a whole is moving. In addition, the atoms possess energy because of the forces between the protons, neutrons, and electrons which make them up. The sum of these energies is the 'internal energy' of the system (represented by the letter 'U'). The word 'internal' is used because these contributions to the energy are from within the system - not from external forces like gravity.

Heat (represented by 'q') is energy transferred into (or out of) the system which increases (or decreases) the random motions of the atoms within the system. So if heat flows in or out of a system, the internal energy of the system will change. Work (represented by 'w') is also a transfer of energy into (or out of) the system, but in this case the energy transferred causes all the atoms in the system to move faster (or slower) in one particular direction, rather than in random directions which are different for each atom. The water molecules in the steam which pushes back the piston in a steam engine all lose some energy of motion along the direction the piston moves because they have pushed back the molecules in the piston as they collided with it. The steam has done some work.

Other state functions

Enthalpy:

H = U + PV

Thermodynamics is a very practical subject and people studying chemical reactions wanted to apply thermodynamics to the reactions they carried out in their laboratories. In the majority of cases these reactions were carried out in open beakers and flasks. People realised that if the volume of the chemicals increased because of the reaction, some of the change in internal energy of the chemicals caused by the reaction would go into work done in pushing back the atmosphere. So the heat produced or consumed in the reaction would not equal the change in internal energy. A new thermodynamic quantity called enthalpy (represented by 'H') was devised which equaled the heat produced or consumed when a reaction was carried out in a vessel open to the atmosphere (that is, the reaction was carried out at constant pressure).

Back to 'Perpetual Motion.'