![]() In a volume-pressure diagram not only the different thermodynamic stats of the process can be shown, but it also offers the ability to determine the work done by the gas clearly. The pressure in this final state is around 2 bar at a gas temperature of 1413 ☌. Finally, the weight is completely lifted at a gas volume of 142.5 ml. The gas pressure must therefore increase steadily as the volume continues to increase. Due to the increasing leverage ratio during lifting, the gas must subsequently apply more and more force on the piston surface in order to lift the weight further. At the same time, this causes the piston to move and the weight to be lifted. Due to the absorbed heat and the associated temperature increase, the pressure now increases. ![]() In this initial state, the gas has a volume of 50 ml, and the temperature is 20 ☌. To explain the pressure curve: At the beginning of the process, the gas pressure is in equilibrium with the ambient air pressure of 1 bar, so that the gas in the cylinder also has a pressure of 1 bar. Figure: Calculation of pressure-volume work as the area under the p(V) curve In order to illustrate such thermodynamic changes of state of the gas, they are often represented in a volume-pressure diagram. During the lifting of the weight, the gas obviously passes through different thermodynamic states characterized by different volumes, pressures and temperatures. On the one hand, heat is supplied to the gas and, on the other hand, work ist done by the gas in order to lift the weight. Let us now look at the process from the point of view of the gas. When the pressure is finally high enough, the gas begins to push out the piston with a certain force. This is done by heating the gas with a Bunsen burner, which increases the temperature of the gas and thus raises the pressure. To be able to lift the weight, the pressure in the cylinder must be increased. Consequently, it is a closed thermodynamic system with the cylinder walls and the piston surface as the system boundary. With the piston rod, which is designed as a rack, a weight is lifted via a gear wheel and a lever. The piston slides friction-free inside the cylinder. In the following, we will consider a horizontally standing cylinder which is closed by a movable piston. Changes of state in volume-pressure diagrams ![]() ![]() This ultimately leads to the concept of pressure-volume work. Therefore, the release or absorption of work during the change of state of a gas will be discussed in more detail in this article. Thus, gases play a central role in both heat engines and heat pumps. Figure: Operating principle of a heat pump (conversion of work into heat) The gas therefore converts work into heat. From the point of view of the gas, therefore, work is done on to the gas on the one hand, while the gas in turn releases heat. If the outlet valve is closed during pumping, not only does the pressure rise very sharply, but an increase in temperature can also be observed due to the work done on the gas during compression. The principle of a heat pump can be illustrated using an air pump. the conversion of work into heat for heating purposes, on the other hand, takes place in so-called heat pumps. Figure: Operating principle of a heat engine (conversion of heat into work) For this reason, internal combustion engines are also generally referred to as heat engines. From the point of view of the gas, therefore, heat is supplied to the gas on the one hand, while on the other hand work is done by the gas. This leads to an increase in pressure in the cylinder, which pushes the piston down and drives the crankshaft. The temperature of the gaseous fuel-air mixture rises due to heat released during the combustion of the fuel. In internal combustion engines, for example, they convert heat into work. Gases play an important role in thermodynamics.
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