Page 65 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

temperature of the fluid? In this case, the fluid cannot be liquefied by an isothermal

compression, i.e. if it is required to condense the fluid, then cooling of the fluid must first

be carried out. In the simple treatment of such fluids, their behavior is likened to that a

perfect gas. Although, strictly speaking, a perfect gas is an ideal which can never be
realized in practice. The behavior of many ‘permanent’ gases, e.g. hydrogen, oxygen, air

etc is very similar to the behavior of a perfect gas to a first approximation.

A perfect gas is a collection of particles that:

❑ are in constant, random motion,
❑ have no intermolecular attractions (which leads to elastic collisions in which no

energy is exchanged or lost),

❑ are considered to be volume-less points.

You are more familiar with the term ‘ideal’ gas. There is actually a distinction between

these two terms but for our purposes, you may consider them interchangeable. The

principle properties used to define the state of a gaseous system are pressure (P), volume
(V) and temperature (T). SI units (Systems International) for these properties are Pascal

3
3
(Pa) for pressure, m for volume (although liters and cm are often substituted), and the
absolute scale of temperature or Kelvin (K).

Two of the laws describing the behavior of a perfect gas are Boyle’s Law and Charles’ Law.

2.4.2 Explain the gas constant, universal gas constant and general gas

equation.

a. Boyle’s Law

The Boyle’s Law may be stated as follows:

Provided the temperature T of a perfect gas remains constant, then volume, V of a given

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