Once the gas has expanded, the pressure will certainly be the same as before because the same free-body diagram applies. If the volume occupied by the gas doubles, how much work has the gas done?Īn assumption to make here is that the pressure is constant. (b) The gas is heated, expanding it and moving the piston up. The pressure in the gas isn't much bigger than atmospheric pressure, just enough to support the weight of the piston. Solving for the pressure of the gas gives: The piston is in equilibrium, so the forces balance. These two forces are balanced by the upward force coming from the gas pressure. The weight of the piston acts down, and the atmosphere exerts a downward force as well, coming from force = pressure x area. This can be determined from a free-body diagram of the piston. The pressure above the piston is atmospheric pressure. The gas is confined by a piston with a weight of 100 N and an area of 0.65 m 2.
An example with numbers might make this clearer.Ĭonsider a gas in a cylinder at room temperature (T = 293 K), with a volume of 0.065 m 3. This is an example of how work is done by a thermodynamic system. If the piston is pushed down, on the other hand, the piston does work on the gas and the gas does negative work on the piston. If the gas is heated, it will expand and push the piston up, thereby doing work on the piston. A good example of a thermodynamic system that can do work is the gas confined by a piston in a cylinder, as shown in the diagram. What does it mean for the system to do work? Work is simply a force multiplied by the distance moved in the direction of the force. We've talked about how heat can be transferred, so you probably have a good idea about what Q means in the first law. Q is positive if heat is added to the system, and negative if heat is removed W is positive if work is done by the system, and negative if work is done on the system. The first law is simply a conservation of energy equation: The first law of thermodynamics relates changes in internal energy to heat added to a system and the work done by a system. In other words, systems at the same temperature will be in thermal equilibrium with each other. This occurs when the systems are at the same temperature. When two systems are in thermal equilibrium, there is no net heat transfer between them. Thermal equilibrium is an important concept in thermodynamics. If the gas is heated, it will expand, doing work on the piston this is one example of how a thermodynamic system can do work. A good example of a thermodynamic system is gas confined by a piston in a cylinder. Thermodynamics is the study of systems involving energy in the form of heat and work. The first law of thermodynamics The first law of thermodynamics
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