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If a gas, such as hydrogen, is pressurized into an air tight container, a force in terms of pascals (or whatever unit you want to use) is exerted, correct? That is what pushes against every surface within the container. But what I don't understand is how the gas can constantly push against the walls without being supplied more energy. Does the force of pressure not need energy, or am I missing something? What about when the force is used to move something, such as in a hydraulics system?

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The gas isn't doing any work (expending energy) because it isn't moving the walls of the container. Each time a gas molecule hits the walls of the container it bounces off imparting a force on the container that balances out the force of the container on the gas. In this way the energy of the gas is conserved. The force in a hydraulic system does push the walls of the container and does a great deal of work. –  Brandon Enright Jan 14 '14 at 5:13
Remember, $W = f \cdot \Delta d$ and when the walls aren't moving $\Delta d = 0$ and therefor no mater how much force $f$ is imparted on the walls, the work $W$ is $0$ too. In hydraulics the wall moves and $\Delta d > 0$ so work is done (energy is expended). –  Brandon Enright Jan 14 '14 at 5:14
The collisions are elastic. –  Ruben Jan 14 '14 at 6:25
@Ruben the collisions don't have to be elastic. As long as the gas in the container and the external environment are in thermal equilibrium the collisions can be inelastic and the energy flowing out of the container do to inelastic collisions will balance out the energy flowing into the container via inelastic collisions. –  Brandon Enright Jan 14 '14 at 7:41
@BrandonEnright Yes, you are right. I did not know that before (the reason being that the education system tells you A this year, and then the next it tells you it is actually B). Thanks for pointing it out. –  Ruben Jan 14 '14 at 12:22

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