Page 9 - 4461

though the bodies have equal values of no other properties. Two bodies may be at the
same temperature although the mass of one is many times that of the other.
It is a matter of experience than two bodies each in temperature (or thermal)
equilibrium with a third body are in temperature equilibrium with each other. (This
statement is sometimes called the zeroth law of thermodynamics.) In view of this fact, it
is possible to determine if two bodies are at the same temperature without bringing them
into contact with each other; it is necessary only to see if they are each in thermal
equilibrium with a third body. The third body is usually what we call a thermometer. A
thermometer is a body which has a readily measurable property which is a function of
temperature. In a mercury-in-glass thermometer, the volume of the mercury depends on
its temperature. In a resistance thermometer, the electrical resistance of the thermometer
element is a temperature-dependent property. In order for a thermometer to indicate the
temperature of another body, the thermometer and the other body must be in contact
with each other long enough and must be sufficiently isolated from other bodies so that
they will attain temperature equilibrium with each other. The temperature of the
thermometer is then the temperature of the other body.

1.3 Equation of the state of an ideal gas
When a system is in thermodynamic equilibrium, then this state can be specified by
some general properties of the system, such as its pressure, volume and temperature.
Equation (5) shows a relation between the three variables P, V and T. If we are given
any two of them, the third one can be determined by using equation (5). Hence, two
variables are enough to determine the state of a system. For example, the equation of
the state of an ideal gas is:

P V 
 const , (1.6)
T

which is also called the gas equation. For each state of the gas, its pressure P, volume V
and temperature T have definite values. If the state of gas is changed, the values of P, V
and T may also change. The volume occupied by a gas at a given pressure and
temperature is proportional to its mass. Thus the constant in Eq.5 must also be
proportional to the mass of the gas. We therefore write the constant in Eq.1.6 as μR,
where μ– is the mass of the gas in moles and R is a different constant for each gas/ R is
called specific gas constant, and R μ is called universal gas constant:

R μ = μR, (1.7)
Joule
where R  8314 ,

mole K
where μ is the mass of the gas in moles or kilogram mole of a substance.

Joule
R  - specific gas constant.
kg  K

Then we write Eq.3 as:
9