CARNOT CYCLE.

Film Loop: Properties of a Gas

Length: 3:18 Min., Black and White, No Sound

In the previous films of this series, we have studied the speeds, free paths,energies, and distributions of pucks on an air table in an attempt to understand the behavior of atoms and molecules in a gas. We have seen that, although this behavior varies randomly from moment to momoent and particle to particle, it does follow a statistically predictable pattern.

In this film, we are no longer interested in the behavior of atoms and molecules as individuals, but rather on the net effect of many particles. It is this group behavior which gives rise to those bulk properties of a gas which we can readily measure - such properties as pressure and temperature. Again, however, because we are actually dealing with a relatively small population of pucks on a two dimensional air table, we can only arrive at conclusions about the properties of real gases by extrapolation.

Pressure When a puck rebounds from the walls, it obviously changes direction and, hence, velocity. Assuming it has mass m and is moving normal to a wall with velocity v, what is the magnitude of its change in momentum? If it hits the wall with a collision frequency f, what is the average force on the wall? If, instead of one puck, there are N pucks moving in all directions, what is the force on the wall? (Hint: On the average, as many pucks are moving up and down as sideways.) If the wall is of length L, what is the force per unit length? This force per unit length in the two dimensional puck gas corresponds to the force per unit area or pressure in a three dimensional volume of real gas.

Effect on Force per unit Length of changes in Number,Area and Vibration Rate Perhaps as you would intuitively expect,increasing the number of pucks, decreasing the area containing the pucks, and increasing the vibration rate of the walls all tend to increase the force per unit length. When we increase the vibration rate, more kinetic energy is imparted to the pucks. In the case of a gas, we call the measure of this energy the temperature. Continuing the analysis begun above, we could arrive at the universal gas law: PV = nRT where nis the number of units of gas and R is a constant relating the units of temperature to those of kinetic energy per unit of gas.

Isothermal and Adiabatic Compression and ExpansionIn isothermal compression and expansion, as the name implies, the temperature remains constant.