Zeroth Law of Thermodynamics,Temperature and it's Measurement

1 Zeroth Law of Thermodynamics,Temperature and it's Measurement

Thermal Equilibrium

A definition of temperature in terms of concepts that are independently defined or accepted as primitive is difficult to give. However, it is possible to arrive at an objective understanding of equality of temperature by using the fact that when the temperature of a body changes, other properties also change.

To illustrate this, consider two copper blocks, and suppose that our senses tell us that one is warmer than the other. If the blocks were brought into contact and isolated from their surroundings, they would interact in a way that can be described as a thermal (heat) interaction.

During this interaction, it would be observed that the volume of the warmer block decreases somewhat with time, while the volume of the colder block increases with time. Eventually, no further changes in volume would be observed, and the blocks would feel equally warm.

Similarly, we would be able to observe that the electrical resistance of the warmer block decreases with time, and that of the colder block increases with time; eventually the electrical resistances would become constant also. When all changes in such observable properties cease, the interaction is at an end. The two blocks are then in thermal equilibrium.

Zeroth law of thermodynamics

 It is a matter of experience that when two bodies are in thermal equilibrium with a third body, they are in thermal equilibrium with one another. This statement, which is sometimes called the Zeroth law of thermodynamics.

 If object A is in thermal equilibrium  with object C, and object B is  separately in thermal equilibrium  with object C, then objects A and  B will be in thermal equilibrium  if they are placed in thermal  contact

Temperatures Scales

Celsius and Fahrenheit scales

  • The lower fixed point is the ice point the temperature at which ice and water coexist at 1 atm. of pressure.
  • The upper fixed point is the steam point, the temperature at which steam and  water coexist at 1 atm. of pressure.

  Celsius and Kelvin scale

  • The Kelvin scale is an absolute thermodynamic temperature scale that provides a continuous definition of temperature, valid over all ranges of temperature. Empirical measures of temperature, with different thermometers, can be related to the Kelvin scale.

    Comparison of two temperature scales

    Quasi static process

      Every moment  in  the  process, system  approaches  to  the  equilibrium  state,  so  that,  within  the            every  little  time dt, the state  parameters  of  every  system  part  have  certain  value,   the   whole   process    is  considered    to   be   composed   by  a   serious   of  states  which  approach  equilibrium  states.

    Processes are sometimes modeled as an idealized type of process called a quasi equilibrium (or quasi static) process. A quasi equilibrium process is one in which the departure from thermodynamic equilibrium is at most infinitesimal. All states through which the system passes in a quasi equilibrium process may be considered equilibrium states. Because non equilibrium effects are inevitably present during actual processes, systems of engineering interest can at best approach, but never realize, a quasi equilibrium process.