We have seen how the internal energy U of a system changes when the energies of constituent particles vary. This change in the internal energy can take place in two different ways. The transfer of energy at a microscopic level occurs when two systems are put in thermal contact, such that the particles of both system may suffer collisions between them. In these collisions the particles with more energy (those of the system with higher temperature) release some energy that is gained by the particles with less average energy (belonging to the system with lower temperature). The transfer of energy ends up when the two systems reach the same temperature, i.e., when their particles have the same average energy. The energy transferred is the heat Q.
On the other hand, we have also seen how a gas expanding against a piston makes a work W, at the expenses of its internal energy U. This can be regarded as a macroscopic transfer of energy. Summarizing the two cases, the conservation of energy is stated in the First Law of Thermodynamics in the form
Some experiences will be
carried out to become familiar with this Law
in various circumstances. First, we shall consider the expansion of a
against a movable wall with no heat exchange with the environment. A
in which there is no heat exchange is called adiabatic.
The values of pressure and volume can be seen in a window during the process, in a typical P-V diagram.
In an adiabatic experience such as that described above,
Press the button "Isothermal" in the control window. Each time a particle collides with the piston the energy loosed by it is released again to the particle so the system does not change their internal energy neither his temperature.
In an isothermal expansion,
WORK IN AN ISOTHERMAL PROCESS
From the state equation of an ideal gas introduced in Section 3 TEMPERATURE, the work done by an ideal gas during an isothermal expansion is given by
where n is the number of moles, and the modified constant appearing in this equation is known as R, the constant of ideal gases.
|5.Heat||6.Work||8.Entropy||9.Velocity Distribution||10.Specific Heat|