5at Train

  The Class 5AT Advanced Technology Steam Locomotive Project   

New Generation Steam -- The Ultimate 4-6-0 -- 380 mile range -- An Engineering Legacy 


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Enthalpy is a “term of convenience” that is useful in the interpretation and application of Thermodynamics.  It is basically a measure of energy.

The term enthalpy is defined as the sum of a system's internal energy plus the product of its pressure and volume, or

H = U + P x V

  • where H is the enthalpy of the gas (in Joules),
  • U is its internal energy (in Joules),
  • P is its pressure in Pascals, and
  • V is its volume in cubic metres.

Commonly the system may be a combination of solids, liquids and gases, in which case most of its internal energy applies to the solid and/or liquid components while the PV term defines the energy of the gaseous component.

Internal energy of a system is usually defined as comprising the combined sensible energy, latent energy, kinetic energy, potential energy, electrical energy and chemical energy of a system's components (see http://en.wikipedia.org/wiki/Internal_energy).

Specific enthalpy (usually denoted by the lower-case letter ‘h’) is the enthalpy per unit of mass, often measured in units of kJ/kg.

Measurement or calculation of a change in enthalpy is usually more meaningful than the value itself.

For further information, see: