Now let`s look at the role of energy in the behavior of gases. When you inflate a bicycle tire by hand, you work by exerting repeated force on a track. This energy is used to increase the air pressure in the tire and increase the temperature of the pump and air. The final temperature is about 6% higher than the original temperature, so the final pressure is also about 6% higher. Note that absolute pressure and absolute temperature must be used in the law of perfect gases. In this equation, it is important to note sign conventions, where a positive value for heat represents Q, heat added to the system, and a positive value for work, W, indicating work on gas. If energy were drawn from the system, such as in the heat taken from the system or from the work done by the system, these amounts would be negative. How many molecules are there in a typical object, for example gasoline in a tire or water in a drink? We can use the ideal gas law to get an idea of the typical size of nitrogen. The law of perfect gases can be considered as another manifestation of the law of conservation of energy (see conservation of energy). Working on a gas results in an increase in its energy, an increase in pressure and/or temperature, or a decrease in volume. This increase in energy can also be thought of as an increase in internal kinetic energy given the atoms and molecules of the gas. Question: Heat is extracted from an ideal gas when its pressure drops from 200 kPa to 100 kPa. The gas then expands from a volume of 0.05 m3 to 0.1 m3, as shown in the PV diagram below.

If the AC curve represents an isotherm, you will find the work of the gas and the heat added to the gas. Question: A liquid is converted to gas at atmospheric pressure (101,325 Pa). The volume of liquid was 5×10-6 m3. The volume of gas is 5×10-3 m3. How much work has been done? Fig. 2: A volume pressure graph for a constant pressure or isobaric process. The area under the curve corresponds to the work of the gas, since W = PΔV. Note that the job calculated in both methods returns the same numeric response (same size) but with different signs. Note that the work is negative, indicating that the gas has worked, which correlates with the expansion of the gas. Law of perfect gases: The physical law that relates the pressure and volume of a gas to the number of gas molecules, or to the number of moles of gas and the temperature of the gas.

The second law of thermodynamics can be expressed in several ways. A statement of this law states that heat flows naturally from a warmer object to a cooler object and cannot flow from a cooler object to a warmer object without working on the system. This can be observed quite easily in everyday circumstances. For example, your cold spoon touching your hot soup will never make your soup hotter and your spoon cooler. In the metric system, the unit of work is the joule, in the English system, the unit is the foot-pound. In general, during a change of state, the volume and pressure change. Therefore, it is more correct to define the work as the integrated variable pressure or added up multiplied by the change in volume from state 1 to state 2. If we use the symbol S [ ] ds for integral, then: The state of a gas is determined by the values of certain measurable properties such as pressure, temperature and the volume that the gas occupies.

The values of these variables and the state of the gas can be changed. In this figure, we show a gas trapped in a blue glass in two different states. On the left, in state 1, the gas has a higher pressure and occupies a smaller volume than in state 2 on the right. We can plot the state of the gas in a pressure diagram based on volume called the p-V diagram, as shown on the right. To change the state of the gas from state 1 to state 2, we must change the conditions in the glass, either by heating the gas, or by physically changing the volume by moving a piston, or by changing the pressure by adding or removing weights from the balloon. In some of these changes we work on the gas or let it act, in other changes we add heat or remove it. Thermodynamics helps us determine the amount of work and the amount of heat needed to change the state of the gas. Note that in this example we have a solid gas mass. We can therefore represent either the physical volume or the specific volume, the volume divided by the mass, since the change is the same for a constant mass.

In the figure, we use physical volume. In a pressure vs volume graph, the job is the area under the curve that describes how the state is changed from state 1 to state 2. It is sometimes convenient to work with a unit other than molecules when measuring the amount of substance. One mole (abbreviated mol) is defined as the amount of a substance containing as many atoms or molecules as atoms are present in exactly 12 grams (0.012 kg) of carbon-12. The actual number of atoms or molecules in a mole is called the Avogadro number (NA) in recognition of the Italian scientist Amedeo Avogadro (1776-1856). He developed the concept of mole, based on the assumption that volumes of gas equal to the same pressure and temperature contain the same number of molecules. That is, the number is independent of the type of gas. This hypothesis has been confirmed, and the value of the Avogadro number is NA = 6.02 × 1023 mol−1. The law of perfect gases can be derived from the basic principles, but was originally derived from the experimental measurements of Charless` law (the volume occupied by a gas is proportional to the temperature at a fixed pressure) and Boyles` law (that for a fixed temperature, the PV product is a constant).

In the ideal gas model, the volume occupied by its atoms and molecules is a negligible fraction of V. The law of perfect gases describes the behavior of real gases under most conditions. (Note, for example, that N is the total number of atoms and molecules, regardless of the type of gas.) A very common expression of the law of perfect gases uses the number of moles n instead of the number of atoms and molecules N. We start from the law of perfect gases, PV = NkT, multiply and divide the equation by the Avogadro number NA. The result is [latex]PV=frac{N}{N_{text{A}}}N_{text{A}}kT[/latex]. Question: Five thousand joules of heat are added to a closed system, which then performs 3000 joules of work. What is the net change in the internal energy of the system? Step 1.Investigate the situation to determine if an ideal gas is involved. Most gases are almost ideal.

If the working force is multiplied by the displacement and compressive force on the surface, the force can be replaced by the pressure multiplied by the surface. The area multiplied by the displacement gives the volume variation of the gas. Because of the sign convention that the work on the gas is positive (corresponding to a decrease in volume), you can write the work as W = -PΔV. For a gas, the work is the product of pressure p and volume V during a volume change. If the process is carried out reversibly, the viscous stresses are negligible and the thermodynamic pressure is uniform throughout the gas, so $$sigma_{vf}=0$$ and $$p_f=p$$where p are determined from the total volume and temperature of the gas, for example from an equation of state such as the law of perfect gases. So, for a reversible process, $$sigma_f=P_{ext}=p$$ and $$W=int{P_{ext}dV}=int{pdV}$$ Here is a link to an article I wrote explaining the difference between reversible and irreversible gas expansion (and compression work) in terms of a close analogy with a mechanical spring damper system: www.physicsforums.com/insights/reversible-vs-irreversible-gas-compressionexpansion-work/ Another statement of this law states: that the level of entropy, or disorder, in a closed system can only increase or remain the same. This means that your office will never naturally become more organized without working. It also means you can`t drop a handful of plastic bricks and watch them spontaneously end up in an impressive model of a medieval castle.