So we might expect that the amount of work done on, or by a gas could be different depending on exactly how the state is changed. As an example, on the graph on the figure, we show a curved black line from State 1 to State 2 of our confined gas.
But the work for the constant pressure process is greater than the work for the curved line process. The work done by a gas not only depends on the initial and final states of the gas but also on the process used to change the state. Different processes can produce the same state, but produce different amounts of work.
In some of these changes, we do work on, or have work done by the gas, in other changes we add, or remove heat. Thermodynamics helps us determine the amount of work and the amount of heat necessary to change the state of the gas. Notice that in this example we have a fixed mass of gas.
At the interface with the surroundings, by Newton's third law, the force per unit area exerted the gas on its surroundings is equal to the pressure of the surroundings on the gas. But, in an irreversible expansion or compression process, the pressure of the gas may not be uniform within the cylinder.
But in the second process, the straight line from State 1 to State "a" and then to State 2, heat was transferred to the gas during the constant pressure process.
The state of a gas is determined by the values of certain measurable properties like the pressure, temperature, and volume which the gas occupies. The values of these variables and the state of the gas can be changed. On this figure we show a gas confined in a blue jar in two different states. On the left, in State 1, the gas is at ...
Thermodynamics helps us determine the amount of work and the amount of heat necessary to change the state of the gas. Notice that in this example we have a fixed mass of gas. We can therefore plot either the physical volume or the specific volume, volume divided by mass, since the change is the same for a constant mass.
Thermodynamics is a branch of physics which deals with the energy and work of a system. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. In aerodynamics, we are most interested in the thermodynamics of high speed flows, and in propulsion systems which produce thrust by accelerating ...
But the work for the constant pressure process is greater than the work for the curved line process. The work done by a gas not only depends on the initial and final states of the gas but also on the process used to change the state. Different processes can produce the same state, but produce different amounts of work.
At the interface with the surroundings, by Newton's third law, the force per unit area exerted the gas on its surroundings is equal to the pressure of the surroundings on the gas. But, in an irreversible expansion or compression process, the pressure of the gas may not be uniform within the cylinder.
The work done by an expanding gas is the energy transferred to its surroundings. In effect, as the gas expands it is compressing its surroundings so the work done is the force exerted on the surroundings (i.e. the pressure of the surroundings times the area) times the distance moved.
The ideal gas law applies only if the gas is at thermodynamic equilibrium. where P int is the pressure of the gas right next to the piston. This is just a mild rephrasing of the definition of work. The work done by the piston on the outside is. where P ext is the pressure of the external air right next to the piston.
If it is free expansion then one molecule hitting the piston will let piston fly away (assuming it is massless). Then the remaining gas molecules expand and we say no work is done by gas. Let's take another example. Piston has mass with gas on one side and vacuum on other side.
In addition, viscous stresses contribute to the force per unit area exerted by the gas at the interface (as well as throughout the cylinder), so the equation of state (e.g., ideal gas law) cannot be used to establish the gas pressure within the cylinder or at the interface. The ideal gas law applies only if the gas is at thermodynamic equilibrium.
In this case the expanding gas does no work regardless of the initial pressure of the gas. At the interface with the surroundings, by Newton's third law, the force per unit area exerted the gas on its surroundings is equal to the pressure of the surroundings on the gas.
Work is done by the gas against the external pressure.If there is a case of free expansion of the gas (as in vacuum) the work done by the gas is zero as no opposing forces are present to prevent expansion of the gas, hence it is evident that work done by the gas is only due to external pressure. If the process is quasistatic (that is ...